Fix crash on dictionary initialization after return to default in C#.

editor/editor_properties_array_dict.cpp

@@ -960,6 +960,7 @@ void EditorPropertyDictionary::update_property() {
 			memdelete(container);
 			button_add_item = nullptr;
 			container = nullptr;
+			add_panel = nullptr;
 			slots.clear();
 		}
 		return;