具有customCell布局的CollectionView不起作用

我正在使用Xamarin.iOS构建应用程序 . 首先，我想在GridView中显示类别列表 . 为此，我使用 CollectionView 与 Custom Cell Layout . 为了构建自定义单元格布局，我在界面中使用了"CollectioViewCell"，这允许我使用拖放设计布局 .

以下是代码：

ViewController.cs

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();

    //initialize the data
    sectionCategories = new List<SectionCategory>();
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));
    sectionCategories.Add(new SectionCategory("Lajme", "LajmeUrl"));

    sectionGrid.RegisterClassForCell(typeof(SectionViewCell), SectionViewCell.Key);
    sectionGrid.Source = new SectionViewSource (sectionCategories);
}

SectionViewCell.cs

public partial class SectionViewCell : UICollectionViewCell
{
    public static readonly NSString Key = new NSString ("SectionViewCell");
    public static readonly UINib Nib;

    static SectionViewCell ()
    {
        Nib = UINib.FromName ("SectionViewCell", NSBundle.MainBundle);
    }

    public SectionViewCell (IntPtr handle) : base (handle)

    {

    }

    public void UpdateCell (string text)
    {
        imageUrl.Text = text;
        sectionName.Text = text;
    }
}

SectionViewCell.designer.cs

[Register ("SectionViewCell")]
partial class SectionViewCell
{
    [Outlet]
    [GeneratedCode ("iOS Designer", "1.0")]
    UILabel imageUrl { get; set; }

    [Outlet]
    [GeneratedCode ("iOS Designer", "1.0")]
    UILabel sectionName { get; set; }

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

SectionViewSource.cs

public class SectionViewSource: UICollectionViewSource
{
    public List<SectionCategory> category { get; set; }

    public SectionViewSource(List<SectionCategory> _category)
    {
        category = _category;
    }

    public override nint NumberOfSections (UICollectionView collectionView)
    {
        return 1;
    }

    public override nint GetItemsCount (UICollectionView collectionView, nint section)
    {
        return category.Count;
    }


    public override UICollectionViewCell GetCell (UICollectionView collectionView, NSIndexPath indexPath)
    {

        var cell = (SectionViewCell)collectionView.DequeueReusableCell (SectionViewCell.Key, indexPath);

        cell.UpdateCell ("lajme");

        return cell;
    }
}

但是当我运行应用程序时它会抛出一个错误：

System.NullReferenceException: Object reference not set to an instance of an object
  at MyIkub.SectionViewCell.UpdateCell (System.String text) [0x00008] in /Users/crs/Projects/MyIkub/MyIkub/SectionViewCell.cs:26
  at MyIkub.SectionViewSource.GetCell (UIKit.UICollectionView collectionView, Foundation.NSIndexPath indexPath) [0x00019] in /Users/crs/Projects/MyIkub/MyIkub/SectionViewSource.cs:33
  at at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00005] in /Users/builder/data/lanes/2761/d7cac503/source/maccore/src/UIKit/UIApplication.cs:77
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0001c] in /Users/builder/data/lanes/2761/d7cac503/source/maccore/src/UIKit/UIApplication.cs:60
  at MyIkub.Application.Main (System.String[] args) [0x00008] in /Users/crs/Projects/MyIkub/MyIkub/Main.cs:12

有什么建议？