Merged @bdisp's latest

UICatalog/Scenarios/BackgroundWorkerCollection.cs

@@ -16,13 +16,14 @@ public class BackgroundWorkerCollection : Scenario
 {
     public override void Init ()
     {
-        var main = Application.Run<OverlappedMain> ();
-
-        //main.Dispose ();
-        Application.Top.Dispose ();
+        Application.Run<OverlappedMain> ().Dispose ();
+        Application.Top?.Dispose ();
 
 #if DEBUG_IDISPOSABLE
-        Debug.Assert (Application.OverlappedChildren.Count == 0);
+        if (Application.OverlappedChildren is { })
+        {
+            Debug.Assert (Application.OverlappedChildren?.Count == 0);
+        }
 #endif
     }