AttachedDropShadowCrashFix

Adding a null reference check to AttachedDropShadow crash issue

Author: LalithaNadimpalli

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

@@ -174,9 +174,12 @@ public AttachedShadowElementContext GetElementContext(FrameworkElement element)
         /// <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;
+                }
             }
         }