Xamarin System.NullReferenceException（对象引用未设置为对象的实例）

Question

我正在使用Xamarin工作室，当我点击图片按钮点击时我的应用程序崩溃了。下面是我的代码，当我点击图片按钮时，我的应用程序崩溃了。

using System;
 using MvvmCross.Binding.BindingContext;
 using MvvmCross.Binding.iOS.Views;
using MvvmCross.iOS.Support.SidePanels;
using MvvmCross.Platform;
using UIKit;
using ViewModelShared;

namespace SelfcareApp.Touch
{

[MvxPanelPresentation(MvxPanelEnum.None, MvxPanelHintType.ActivePanel, true, MvxSplitViewBehaviour.Detail)]
public partial class CcFnfView : BaseViewController<CcFnfViewViewModel>
{

    private CcFnfViewViewModel PageViewModel
    {
        get { return (CcFnfViewViewModel)base.ViewModel; }
    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
    //  PageViewModel.ShowMenu();

        var cellWidth = (UIScreen.MainScreen.Bounds.Width - 4 - 8) / 2;
        var cellheight = (UIScreen.MainScreen.Bounds.Height - 130) / 2;

        var headerView = new MobileNumberHeaderView(new CoreGraphics.CGRect(0, 0, UIScreen.MainScreen.Bounds.Width, 50));


        AccountCollectionView.BackgroundColor = UIColor.Clear;

        AccountCollectionView.CollectionViewLayout = new UICollectionViewFlowLayout() { ItemSize = new CoreGraphics.CGSize(cellWidth, cellheight), MinimumLineSpacing = 4, MinimumInteritemSpacing = 4 };
        AccountCollectionView.RegisterNibForCell(TileViewCell.Nib, TileViewCell.Key);
        var homeSource = new MvxCollectionViewSource(AccountCollectionView, TileViewCell.Key);
        AccountCollectionView.Source = homeSource;
        //AudioAlbumsCollectionView.


        var set = this.CreateBindingSet<CcFnfView, CcFnfViewViewModel>();
        set.Bind(headerView.LabelFiled).To(vm => vm.DefaultPhone).WithConversion("MSISDN");;
        set.Bind(headerView.Button).To(vm => vm.SelectAccountCommand);
        set.Bind(headerView.Button).For(x => x.Hidden).To(vm => vm.IsChangeNumberButtonVisible).WithConversion("BooleanReverse");
        set.Bind(Title).To(vm => vm.PageTitle);
        set.Bind(homeSource).To(vm => vm.MenuItems);
        set.Bind(homeSource).For(t => t.SelectionChangedCommand).To(vm => vm.MenuSelectedCommand);
        set.Bind(this).For(t => t.Title).To(vm => vm.PageTitle);
        set.Bind(Progress).For(t => t.Visible).To(vm => vm.ProgressBarVisibility);
        set.Apply();


        Add(headerView);

    }
    public override void ViewWillAppear(bool animated)
    {
        base.ViewWillAppear(animated);

        if (NavigationController != null)
            NavigationController.NavigationBarHidden = false;

        var sideMenu = Mvx.Resolve<IMvxSideMenu>();
        sideMenu.Close();
    }
 }
}

Designer.cs：

  using Foundation;
   using System;
  using System.CodeDom.Compiler;
 using UIKit;

  namespace SelfcareApp.Touch
{
[Register ("CcFnfView")]
partial class CcFnfView
{
    [Outlet]
    UIKit.UICollectionView AccountCollectionView { get; set; }

    void ReleaseDesignerOutlets ()
    {
        if (AccountCollectionView != null) {
            AccountCollectionView.Dispose ();
            AccountCollectionView = null;
        }
    }
}
}

应用程序在该行崩溃：

AccountCollectionView.BackgroundColor = UIColor.Clear;