Fixes NullReferenceException in AdornmentView.Contains when Adornment is null (#4884)

AdornmentView has a parameter-less constructor (for AllViewsTester reflection
support) that leaves Adornment = null. When such a view is placed inside a
SuperView (e.g. the Themes UICatalog scenario creates AdornmentView/MarginView
via reflection), moving the mouse over it caused a NullReferenceException at
Adornment!.Thickness.Contains(...) because the null-forgiving ! operator did
not prevent the dereference.

Fix: add a guard clause returning false when Adornment is null, matching the
existing pattern used in OnClearingViewport, OnDrawingText, and
OnDrawingSubViews.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tig <tig@users.noreply.github.com>

Author: 3 people

Terminal.Gui/ViewBase/Adornment/AdornmentView.cs

@@ -190,10 +190,15 @@ public override bool Contains (in Point location)
             }
         }
 
+        if (Adornment is null)
+        {
+            return false;
+        }
+
         Rectangle outside = Frame;
         outside.Offset (parentOrSuperView.Frame.Location);
 
-        return Adornment!.Thickness.Contains (outside, location);
+        return Adornment.Thickness.Contains (outside, location);
     }
 
     #endregion View Overrides

Tests/UnitTestsParallelizable/ViewBase/Adornment/AdornmentViewTests.cs

@@ -0,0 +1,25 @@
+namespace ViewBaseTests.Adornments;
+
+// Copilot
+public class AdornmentViewTests
+{
+    /// <summary>
+    ///     Regression test for https://github.com/gui-cs/Terminal.Gui/issues/4883.
+    ///     AdornmentView.Contains must not throw NullReferenceException when Adornment is null
+    ///     (i.e. the view was created via its parameter-less constructor, as happens when
+    ///     AllViewsTester / Themes UICatalog scenario creates it via reflection).
+    /// </summary>
+    [Fact]
+    public void Contains_Returns_False_When_Adornment_Is_Null ()
+    {
+        // Arrange - parameter-less ctor leaves Adornment = null
+        AdornmentView adornmentView = new ();
+
+        // Act & Assert - must not throw NullReferenceException
+        bool result = adornmentView.Contains (new Point (0, 0));
+
+        Assert.False (result);
+    }
+
+
+}