Fix ThicknessTests.

UnitTests/Drawing/ThicknessTests.cs

@@ -177,8 +177,9 @@ public class ThicknessTests
         // Add a frame so we can see the ruler
         var f = new FrameView { X = 0, Y = 0, Width = Dim.Fill (), Height = Dim.Fill () };
 
-        Application.Top.Add (f);
-        Application.Begin (Application.Top);
+        var top = new Toplevel ();
+        top.Add (f);
+        Application.Begin (top);
 
         ((FakeDriver)Application.Driver).SetBufferSize (45, 20);
         var t = new Thickness (0, 0, 0, 0);