Throw exception when ViewCell View is null (#752)

Xamarin.Forms.Platform.Android/Cells/ViewCellRenderer.cs

@@ -2,6 +2,7 @@ using Android.Content;
 using Android.Views;
 using AView = Android.Views.View;
 using Xamarin.Forms.Internals;
+using System;
 
 namespace Xamarin.Forms.Platform.Android
 {
@@ -32,6 +33,9 @@ namespace Xamarin.Forms.Platform.Android
 				rowHeight = ListView.RowHeightProperty;
 			}
 
+			if (cell.View == null)
+				throw new InvalidOperationException($"ViewCell must have a {nameof(cell.View)}");
+
 			IVisualElementRenderer view = Platform.CreateRenderer(cell.View);
 			Platform.SetRenderer(cell.View, view);
 			cell.View.IsPlatformEnabled = true;

Xamarin.Forms.Platform.iOS/Cells/ViewCellRenderer.cs

@@ -139,6 +139,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 			IVisualElementRenderer GetNewRenderer()
 			{
+				if (_viewCell.View == null)
+					throw new InvalidOperationException($"ViewCell must have a {nameof(_viewCell.View)}");
+
 				var newRenderer = Platform.CreateRenderer(_viewCell.View);
 				_rendererRef = new WeakReference<IVisualElementRenderer>(newRenderer);
 				ContentView.AddSubview(newRenderer.NativeView);