Merge pull request #4757 from CommunityToolkit/PR4750Fix

AttachedDropShadowCrashFix

Microsoft.Toolkit.Uwp.UI/Shadows/AttachedShadowBase.cs

@@ -174,9 +174,12 @@ namespace Microsoft.Toolkit.Uwp.UI
         /// <inheritdoc/>
         public IEnumerable<AttachedShadowElementContext> EnumerateElementContexts()
         {
-            foreach (var kvp in ShadowElementContextTable)
+            if (ShadowElementContextTable != null)
             {
-                yield return kvp.Value;
+                foreach (var kvp in ShadowElementContextTable)
+                {
+                    yield return kvp.Value;
+                }
             }
         }