Fixes build warnings recently introduced (#4506)

* Fixed warnings I recently introduced.

Replaced nullable App references with null-forgiving operator (!) in MessageBox and dialog calls to suppress nullable warnings. Updated XML docs to use Dim.Fill() and clarified method references. Made Application.Popover registration null-safe. Fixed test output helper usage in ShadowTests. No functional changes.

* Fixes Release build warnings

Tests/UnitTests/Application/ApplicationPopoverTests.cs

@@ -331,9 +331,12 @@ public class ApplicationPopoverTests
         public List<Key> HandledKeys { get; } = [];
         public int NewCommandInvokeCount { get; private set; }
 
+#if DEBUG_IDISPOSABLE
         // NOTE: Hides the base DisposedCount property
         public new int DisposedCount { get; private set; }
-
+#else
+        public int DisposedCount { get; private set; }
+#endif
         public PopoverTestClass ()
         {
             CanFocus = true;