NullReferenceException，即使控件存在于前一页

每当我尝试在bedCount()方法中检索上一页下拉列表控件时，它会给出此异常，而我向您保证，我正在搜索的控件非常存在于上一页中。这是什么原因呢?我的代码如下:

public partial class Room2 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    {
        DropDownList[] adult=new DropDownList[6];
        DropDownList[] child = new DropDownList[6];
        TableRow[] trp = new TableRow[5];
        TableRow[] trc ={ trc1, trc2, trc3, trc4, trc5 };
        DropDownList[] rtype ={ DropDownList1,DropDownList2, DropDownList3, DropDownList4, DropDownList5, DropDownList6 };
        Label[] bed ={Label1,Label2,Label3,Label4,Label5,Label6};
        int i,x,c=2;
        for (i = 0; i < 5; i++)
        {
            trp[i] = (TableRow)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("tr" + (i + 1));
        }
        for (i = 0; i < 6; i++,c+=4)
        {
            adult[i] = (DropDownList)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("DropDownList" + c++);
            child[i] = (DropDownList)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("DropDownList" + c);
        }
        for (i = 0; i < 6; i++)
        {
            x = adult[i].SelectedIndex + child[i].SelectedIndex;
            switch (x)
            {
                case 0:
                case 1:
                    rtype[i].Items.Add("Executive Class");
                    rtype[i].Items.Add("Business Class");
                    rtype[i].Items.Add("Gold Class (Type-I)");
                    rtype[i].Items.Add("Gold Class (Type-II)");
                    rtype[i].SelectedIndex = 0;
                    bed[i].Text = "No";
                    break;
                case 2:
                    rtype[i].Items.Add("Business Class");
                    rtype[i].Items.Add("Gold Class (Type-I)");
                    rtype[i].Items.Add("Gold Class (Type-II)");
                    rtype[i].SelectedIndex = 0;
                    bed[i].Text = "1";
                    break;
                case 3:
                    bed[i].Text = "1";
                    goto case 4;
                case 4:
                    rtype[i].Items.Add("Gold Class (Type-I)");
                    rtype[i].Items.Add("Gold Class (Type-II)");
                    rtype[i].SelectedIndex = 0;
                    bed[i].Text = "2";
                    break;
                case 5:
                    bed[i].Text = "2";
                    goto case 6;
                case 6:
                    rtype[i].Items.Add("Gold Class (Type-II)");
                    rtype[i].SelectedIndex = 0;
                    bed[i].Text = "3";
                    break;
            }
            if (i<5 && trp[i].Visible)
                trc[i].Visible = true;
        }
    }
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    if(DropDownList1.Items.Count<=3)
        bedCount(DropDownList1,Label1,0);
}
protected void bedCount(DropDownList d,Label l,int a)
{
    protected void bedCount(DropDownList d,Label l,int x)
{
    DropDownList a=new DropDownList();
    DropDownList c = new DropDownList();
    int s;
    a = (DropDownList)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("DropDownList"+x++);//gives exception here
    c = (DropDownList)PreviousPage.Master.FindControl("ContentPlaceHolder1").FindControl("DropDownList"+x);
    s = c.SelectedIndex + c.SelectedIndex;
    if (d.SelectedItem.Equals("Business Class"))
        if(s==2)
            l.Text = "1";
        else 
            l.Text = "No";
    else if(d.SelectedItem.Equals("Gold Class (Type-I)"))
        if(s==3)
            l.Text = "1";
        else if(s==4)
            l.Text = "2";
        else
            l.Text = "No";
    else if(d.SelectedItem.Equals("Gold Class (Type-II)"))
        if(s==4)
            l.Text = "1";
        else if(s==5)
            l.Text = "2";
        else if(s==6)
            l.Text = "3";
        else
            l.Text = "No";
}

DropDownList[] adult=new DropDownList[6];
DropDownList[] child = new DropDownList[6];