Fix dragging of Elements wich are not contained in the Content Property of their Container

WpfDesign.Designer/Project/Extensions/DefaultPlacementBehavior.cs

@@ -100,18 +100,18 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 		}
 
 		public virtual void LeaveContainer(PlacementOperation operation)
-		{
-			if (ExtendedItem.ContentProperty.IsCollection)
-			{
-				foreach (var info in operation.PlacedItems)
-				{
-					ExtendedItem.ContentProperty.CollectionElements.Remove(info.Item);
-				}
+		{
+			foreach (var info in operation.PlacedItems) {
+				var parentProperty = info.Item.ParentProperty;
+				if (parentProperty.IsCollection)
+				{
+					parentProperty.CollectionElements.Remove(info.Item);
+				}
+				else
+				{
+					parentProperty.Reset();
+				}
 			}
-			else
-			{
-				ExtendedItem.ContentProperty.Reset();
-			}
 		}
 
 		private static InfoTextEnterArea infoTextEnterArea;