Mono For Android: java.lang.NullPointerException on StartAct

我有一个关于StartActivityForResult的问题。我有一个来自网络服务的表单，偶尔会产生一个字段，需要打开凸轮拍照，将其显示在视图上并将其发送回去。

问题是我总是在我的StartActivityForResult上收到一个非描述性的Java.Lang.NullPointerException错误。

这是代码

PrevzemVozila.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using System.Data;
using Android.Content.PM;
using Android.Provider;
namespace UUIDev
{
[Activity(Label = "Prevzem vozila v servis", NoHistory = true, Icon="@drawable/Icon",     ConfigurationChanges = Android.Content.PM.ConfigChanges.Orientation)]
public class PrevzemVozila : Activity
{
    si.rcg.hat._hat_Tablet chk = new si.rcg.hat._hat_Tablet();
    Java.IO.File _file;
    PackageManager _pm;
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.Prevzem);
        _pm = this.PackageManager;
        //Window.SetSoftInputMode(SoftInput.StateAlwaysVisible);
        var Ticket = Intent.GetStringExtra("sentTicket") ?? "";
        var FormIdStr = Intent.GetStringExtra("sentFormId") ?? "";
        var EventNoStr = Intent.GetStringExtra("sentEventNo") ?? "";
        int FormId = int.Parse(FormIdStr);
        int EventNo = int.Parse(EventNoStr);
        DataSet GetSingleEvent = new DataSet();
        GetSingleEvent = chk.GetSingleEvent(Ticket, FormId, EventNo);
        IList<Prevzem> listPrevzem = new JavaList<Prevzem>();

        var lstPrevzem = FindViewById<ListView>(Resource.Id.lstPrevzem);
        var prevzemAdapter = new PrevzemAdapter(this, Resource.Layout.ListPrevzem, new JavaList<Prevzem>());
        lstPrevzem.Adapter = prevzemAdapter;
        foreach (DataRow row in GetSingleEvent.Tables[0].Rows)
        {
            var item = new Prevzem();
            item.IdRec = row["IdRec"].ToString();
            if (item.IdRec.Equals(""))
                item.IdRec = null;
            item.FieldId = row["FieldId"].ToString();
            if (item.FieldId.Equals(""))
                item.FieldId = null;
            item.FieldDesc = row["FieldDesc"].ToString();
            if (item.FieldDesc.Equals(""))
                item.FieldDesc = null;
            item.FieldType = row["FieldType"].ToString();
            if (item.FieldType.Equals(""))
                item.FieldType = null;
            item.isReadOnly = row["isReadOnly"].ToString();
            if (item.isReadOnly.Equals(""))
                item.isReadOnly = null;
            item.FieldValue = row["FieldValue"].ToString();
            if (item.FieldValue.Equals(""))
                item.FieldValue = null;
            item.FieldTextValue = row["FieldTextValue"].ToString();
            if (item.FieldTextValue.Equals(""))
                item.FieldTextValue = null;
            item.OptBox_Options = row["OptBox_Options"].ToString();
            if (item.OptBox_Options.Equals(""))
                item.OptBox_Options = null;

            listPrevzem.Add(item);
            RunOnUiThread(() => {prevzemAdapter.Add(item);});
        }
    }
    public override void OnConfigurationChanged(Android.Content.Res.Configuration newConfig)
    {
        base.OnConfigurationChanged(newConfig);
    }
    public void OnGetCam(Intent _intent, IList<ResolveInfo> _avalableActivities)
    {
        if (_avalableActivities != null && _avalableActivities.Count > 0)
        {
            var dir = new Java.IO.File(
                Android.OS.Environment.GetExternalStoragePublicDirectory(
                Android.OS.Environment.DirectoryPictures), "RCG_Foto");
            if (!dir.Exists())
            {
                dir.Mkdirs();
            }
            _file = new Java.IO.File(dir, String.Format("avto_{0}.jpg", Guid.NewGuid()));
            _intent.PutExtra(MediaStore.ExtraOutput, Android.Net.Uri.FromFile(_file));
            this.StartActivityForResult(_intent, 5);
        }
    }
        protected override void OnActivityResult(int requestCode, Result resultCode, Android.Content.Intent data)
        {
            base.OnActivityResult(requestCode, resultCode, data);
        var imageView = FindViewById<ImageView>(Resource.Id.imageSending);
            // make it available in the gallery
            var mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
            var contentUri = Android.Net.Uri.FromFile(_file);
            mediaScanIntent.SetData(contentUri);
            SendBroadcast(mediaScanIntent);
            // display in ImageView
            var fotka = MediaStore.Images.Media.GetBitmap(ContentResolver, contentUri);
            imageView.SetImageBitmap(fotka);
            fotka.Dispose();
        }
    }
}

PrevzemAdapter.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.IO;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using System.Data;
using Android.Text;
using Android.Content.PM;
using Android.Provider;
namespace UUIDev
{
    [Activity(Label = "Prevzem Adapter")]
    public class PrevzemAdapter : ArrayAdapter<Prevzem>
    {
        int resource;
        PrevzemVozila pa;
        Intent availIntent;
        IList<ResolveInfo> availableActivities;

        Context p_context = null;
        //Initialize adapter
        public PrevzemAdapter(Context context, int resource, IList<Prevzem> items)
            : base(context, resource, items)
        {
            this.resource = resource;
            this.p_context = context;
            this.AvailIntent = new Intent(MediaStore.ActionImageCapture);
            this.AvailActivities = p_context.PackageManager.QueryIntentActivities(AvailIntent, PackageInfoFlags.MatchDefaultOnly);
        }
        public override View GetView(int position, View convertView, ViewGroup parent)
        {
            LinearLayout view;
            //Get our items
            var item = GetItem(position);
            //Inflate the view
            if (convertView == null)
            {
                view = new LinearLayout(Context);
                var inflater = Context.LayoutInflaterService;
                LayoutInflater vi;
                vi = (LayoutInflater)Context.GetSystemService(inflater);
                vi.Inflate(resource, view, true);
            }
            else
            {
                view = (LinearLayout)convertView;
            }
            var FieldDesc = view.FindViewById<TextView>(Resource.Id.txtFieldDesc);
            FieldDesc.Text = item.FieldDesc != null ? item.FieldDesc : "";
            var IdRec = view.FindViewById<TextView>(Resource.Id.txtIdRec);
            IdRec.Text = item.IdRec != null ? item.IdRec : "";
            var FieldId = view.FindViewById<TextView>(Resource.Id.txtFieldId);
            FieldId.Text = item.FieldId != null ? item.FieldId : "";
            var FieldTextValue = view.FindViewById<EditText>(Resource.Id.editVal);
            FieldTextValue.Text = item.FieldTextValue != null ? item.FieldTextValue : "";
            var ButtonValue = view.FindViewById<Button>(Resource.Id.button1);
            ButtonValue.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;
            var CamImage = view.FindViewById<ImageView>(Resource.Id.imageSending);
            CamImage.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;
            var SpinnerValue = view.FindViewById<Spinner>(Resource.Id.spinnerVal);

            bool isReadOnly = bool.Parse(item.isReadOnly);
            if (isReadOnly == true)
            {
                FieldTextValue.Enabled = false;
                SpinnerValue.Enabled = false;

            }
            else
            {
                FieldTextValue.Enabled = true;
                SpinnerValue.Enabled = true;
            }
            // if field is TextBOX_INT
            if (item.FieldType == "TextBOX_INT")
            {
                FieldTextValue.InputType = InputTypes.ClassNumber;   
            }
            // if field is TextBOX or TextBOX_INT or TextAREA
            if (item.FieldType == "TextBOX" || item.FieldType == "TextBOX_INT" || item.FieldType == "TextAREA")
            {
                FieldTextValue.SetSingleLine(true);
                FieldTextValue.Visibility = view.Visibility == ViewStates.Visible ? ViewStates.Visible : ViewStates.Visible;
                SpinnerValue.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;
            }
            // if field is TextAREA
            if (item.FieldType == "TextAREA")
            {
                FieldTextValue.InputType = InputTypes.ClassText;
                FieldTextValue.SetSingleLine(false);
            }
            // if field is ComboBOX or OptionBox
            if (item.FieldType == "ComboBOX" || item.FieldType == "OptionBOX")
            {
                FieldTextValue.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;
                SpinnerValue.Visibility = view.Visibility == ViewStates.Visible ? ViewStates.Visible : ViewStates.Visible;
                List<string> entries = new List<string>();
                String rawXML = item.OptBox_Options;
                StringReader stream = null;
                XmlTextReader reader = null;
                DataSet xmlDS = new DataSet();
                stream = new StringReader(rawXML);
                // Load the XmlTextReader from the stream
                reader = new XmlTextReader(stream);
                xmlDS.ReadXml(reader);
                DataSet myOPTvalues = new DataSet();
                myOPTvalues = xmlDS;
                foreach (DataRow row in myOPTvalues.Tables[0].Rows)
                {
                    var optItem = new PrevzemSpin();
                    optItem.FieldValue = row["FieldValue"].ToString();
                    if (optItem.FieldValue.Equals(""))
                        optItem.FieldValue = null;
                    optItem.FieldTextValue = row["FieldTextValue"].ToString();
                    if (optItem.FieldTextValue.Equals(""))
                        optItem.FieldTextValue = null;

                    entries.Add(optItem.FieldTextValue);
                    SpinnerValue.Tag = optItem.FieldValue;
                }
                var adapter = new ArrayAdapter<string>(p_context, Android.Resource.Layout.SelectDialogItem, entries);
                adapter.SetDropDownViewResource(Android.Resource.Layout.SelectDialogSingleChoice);
                SpinnerValue.Adapter = adapter;
                if (SpinnerValue.Adapter.Count.Equals(2))
                {
                    SpinnerValue.SetSelection(1);
                }                
            }
            // if field is Picture_REQ
            //if (item.FieldType == "Picture_REQ")
            // {
                ButtonValue.Visibility = view.Visibility == ViewStates.Visible ? ViewStates.Visible : ViewStates.Visible;
                CamImage.Visibility = view.Visibility == ViewStates.Visible ? ViewStates.Visible : ViewStates.Visible;
                FieldTextValue.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;
                SpinnerValue.Visibility = view.Visibility == ViewStates.Gone ? ViewStates.Gone : ViewStates.Gone;

                ButtonValue.Click += delegate
                {
                    pa = new PrevzemVozila();
                    if (pa != null)
                    {
                        pa.OnGetCam(this.AvailIntent, this.AvailActivities);
                    }
                };
            //}
            return view;
        }
        public IList<ResolveInfo> AvailActivities
        {
            set { this.availableActivities = value; }
            get { return this.availableActivities; }
        }
        public Intent AvailIntent
        {
            set { this.availIntent = value; }
            get { return this.availIntent; }
        }
    }
}