Xamarin Sqlite-Net插入(对象引用未设置为对象的实例)

我有一个很奇怪的问题，我不明白。我不能插入一个新的项目到我的Sqlite数据库。我一直收到这个错误:-

System.NullReferenceException: Object reference not set to an instance of an object.

我真的不知道为什么，在应用程序中有完全相同的代码工作和保存其他表。

User.cs

public class User
{
    [PrimaryKey, AutoIncrement]
    public int ID { get; set; }
    public string Name { get; set; }
    public string EmailAddress { get; set; }
    public User()
    {
        this.Name = " ";
        this.EmailAddress = " ";
    }
}

UserManager.cs

public class UserManager 
{
    UserRepository repository; 
    public UserManager(SQLiteConnection conn)
    {
        repository = new UserRepository (conn);
    }
    public User GetDriver(int id)
    {
        return repository.GetDriver(id);
    }
    public IList<User> GetDrivers()
    {
        return new List<User>(repository.GetDrivers());
    }
    public int SaveDriver(User item)
    {
        return repository.SaveDriver(item);
    }
    public int DeleteDriver(int id)
    {
        return repository.DeleteDriver(id);
    }
}

UserRepository.cs

public class UserRepository
{
    UserDatabase db = null;
    public UserRepository(SQLiteConnection conn)
    {
        db = new UserDatabase(conn);
    }
    public User GetDriver(int id)
    {
        return db.GetDriverItem(id);
    }
    public IEnumerable<User> GetDrivers()
    {
        return db.GetAllDriverItems();
    }
    public int SaveDriver(User item)
    {
        return db.SaveDriverItem(item);
    }
    public int DeleteDriver(int id)
    {
        return db.DeleteDriverItem(id);
    }
}

UserDatabase.cs

public class UserDatabase
{
    static object locker = new object();
    public SQLiteConnection database;
    public string path;
    public UserDatabase(SQLiteConnection conn)
    {
        database = conn;
        database.CreateTable<User>();
    }
    public IEnumerable<User> GetAllDriverItems () {
        lock (locker) {
            return (from i in database.Table<User>() select i).ToList();
        }
    }
    public User GetDriverItem (int id) 
    {
        lock (locker) {
            return database.Table<User>().FirstOrDefault(x => x.ID == id);
        }
    }
    public int SaveDriverItem (User item) 
    {
        lock (locker) {
            if (item.ID != 0) {
                database.Update(item);
                return item.ID;
            } else {
                return database.Insert(item);
            }
        }
    }
    public int DeleteDriverItem(int id) 
    {
        lock (locker) {
            return database.Delete<User>(id);
        }
    }
}

Application.cs (Android)

public class UserApp : Application
{
    public static UserApp Current { get; private set; }
    public UserManager UserManager { get; set; }
    SQLiteConnection conn;
    public UserApp(IntPtr handle, global::Android.Runtime.JniHandleOwnership transfer)
        : base(handle, transfer)
    {
        Current = this;
    }
    public override void OnCreate()
    {
        base.OnCreate();
        var sqliteFilename = "UserItemDB.db3";
        string libraryPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
        var path = Path.Combine(libraryPath, sqliteFilename);
        conn = new SQLiteConnection(new SQLite.Net.Platform.Generic.SQLitePlatformGeneric(), path);
        UserManager = new UserManager(conn);
        User testUser= new User();
        testUser.Name = "Dave";
        testUser.EmailAddress = "Dave@Gmail.com";
        UserApp.Current.UserManager.SaveDriver(testUser);
    }

这是我的堆栈跟踪:-

System.NullReferenceException: Object reference not set to an instance of an object.
08-09 17:07:32.423 E/mono    (31670):   at     System.Reflection.MonoProperty.GetValue (System.Object obj, System.Object[]     index) [0x00031] in <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.TableMapping+Column.GetValue (System.Object obj) [0x00000] in     <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.SQLiteConnection.Insert (System.Object obj, System.String extra,     System.Type objType) [0x00098] in <filename unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     SQLite.Net.SQLiteConnection.Insert (System.Object obj) [0x00012] in <filename     unknown>:0 
08-09 17:07:32.423 E/mono    (31670):   at     Project.DataLayer.UserDatabase.SaveDriverItem (Project.BusinessLayer.Models.User     item) [0x00038] in     C:'Users'phill'Desktop'Work'ProjectTEST'Project'Project.Shared'DataLayer'UserDat    abase.cs:42 
08-09 17:07:32.423 E/mono    (31670):   at     Project.DataAccessLayer.UserRepository.SaveDriver     (Project.BusinessLayer.Models.User item) [0x00001] in     C:'Users'phill'Desktop'Work'ProjectTEST'Project'Project.Shared'DataAccessLayer'U    serRepository.cs:30 
08-09 17:07:32.423 E/mono    (31670):   at     Project.BusinessLayer.Managers.UserManager.SaveDriver     (Project.BusinessLayer.Models.User item) [0x00001] in     C:'Users'phill'Desktop'Work'ProjectTEST'Project'Project.Shared'BusinessLayer'Man    agers'UserManager.cs:33 
08-09 17:07:32.423 E/mono    (31670):   at Project.Droid.UserApp+    <getDriverDetails>d__16.MoveNext () [0x000e2] in     C:'Users'phill'Desktop'Work'ProjectTEST'Project'Project.Droid'Application.cs:75