[Debugger] When a CellView gets moved to a null Superview, don't UpdateContents()

When the Superview is null, it means that we are being removed from view
so it is unnecessary to UpdateContents(). It can also mean we are in the
process of being disposed. Either way, don't UpdateContents().

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1006517/

Author: jstedfast

main/src/addins/MonoDevelop.Debugger/MonoDevelop.Debugger/ObjectValue/Mac/MacDebuggerObjectCellViewBase.cs

@@ -64,7 +64,7 @@ public MacObjectValueTreeView TreeView {
 		public override NSObject ObjectValue {
 			get { return base.ObjectValue; }
 			set {
-				var target = ((MacObjectValueNode)value)?.Target;
+				var target = ((MacObjectValueNode) value)?.Target;
 
 				if (Node != target) {
 					if (target != null)
@@ -172,7 +172,9 @@ protected void UpdateFont (NSControl control, int sizeDelta = 0)
 		public override void ViewDidMoveToSuperview ()
 		{
 			base.ViewDidMoveToSuperview ();
-			UpdateContents ();
+
+			if (Superview != null)
+				UpdateContents ();
 		}
 
 		public override NSBackgroundStyle BackgroundStyle {