[Debugger] Make sure colorPreview has a non-null Layer property

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1004949/
(I hope)

Author: jstedfast

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

@@ -67,7 +67,8 @@ public MacDebuggerObjectValueView (MacObjectValueTreeView treeView) : base (tree
 			};
 
 			colorPreview = new NSView (new CGRect (0, 0, ImageSize, ImageSize)) {
-				TranslatesAutoresizingMaskIntoConstraints = false
+				TranslatesAutoresizingMaskIntoConstraints = false,
+				WantsLayer = true
 			};
 
 			valueButton = new NSButton {