Enable nullable types and add debugging utilities

Examples/UICatalog/Scenarios/Scrolling.cs

@@ -1,5 +1,7 @@
 #nullable enable
 
+using System.Diagnostics;
+
 namespace UICatalog.Scenarios;
 
 [ScenarioMetadata ("Scrolling", "Content scrolling, IScrollBars, etc...")]
@@ -100,6 +102,8 @@ public class Scrolling : Scenario
         app.Dispose ();
         Application.Shutdown ();
 
+        Debug.Assert (_progressTimer is null);
+
         return;
 
         void AppOnInitialized (object? sender, EventArgs e)