Xamarin FindViewById NullReference异常

Question

我有一个问题，每当我尝试将一个事件处理程序添加到一个按钮时我得到一个空引用异常，我试图创建一个带有DialogFragment的弹出窗口，其中我调用的视图PopUpWindow将显示在屏幕上，但是当我尝试通过id访问按钮并为其分配事件处理程序时，例如：

    Button btnCopyText = dp.view.FindViewById<Button>(Resource.Id.btnCopyText);
    btnCopyText.Click += BtnCopyText_Click;

然后我得到一个空引用异常，任何人都可以帮助我，下面是必要的代码。

class dialog_Popup:DialogFragment
{
    public View view;
    public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        base.OnCreateView(inflater, container, savedInstanceState);
        view = inflater.Inflate(Resource.Layout.PopupWindow, container, false);

        return view;
    }
    public override void OnActivityCreated(Bundle savedInstanceState)
    {
        Dialog.Window.RequestFeature(WindowFeatures.NoTitle);
        base.OnActivityCreated(savedInstanceState);

    }

public class MainActivity : Activity
{
    protected override void OnCreate(Bundle bundle)
    {
     //some code
    }
    public string itemclicked;
    dialog_Popup dp;


    private void Lv_ItemLongClick(object sender, AdapterView.ItemLongClickEventArgs e)
    {
        //View popUpView = LayoutInflater.Inflate(Resource.Layout.PopupWindow,
        //null); // inflating popup layout

        Button height = FindViewById<Button>(Resource.Id.btnCopyText);
        //Then: change the width of the button
        FragmentTransaction transaction = FragmentManager.BeginTransaction();
        dp = new dialog_Popup();
        dp.Show(transaction,"Popup");

        itemclicked = lv.GetItemAtPosition(e.Position).ToString();

        Button btnCopyText = dp.view.FindViewById<Button>(Resource.Id.btnCopyText);
        btnCopyText.Click += BtnCopyText_Click;
        Button btnSaveCurrentAya = dp.view.FindViewById<Button>(Resource.Id.btnSaveCurrentAya);
        btnSaveCurrentAya.Click += BtnSaveCurrentAya_Click;
        Button btnsavingsAya = dp.view.FindViewById<Button>(Resource.Id.savingsAya);
        btnsavingsAya.Click += BtnsavingsAya_Click;*

        Button btnShareFB = dp.view.FindViewById<Button>(Resource.Id.fbShare);
        btnShareFB.Click += BtnShareFB_Click;
    }

}