Fix being able to delete new key/value from dictionary editor.

editor/editor_properties_array_dict.cpp

@@ -744,6 +744,7 @@ void EditorPropertyDictionary::_change_type(Object *p_button, int p_index) {
 	Button *button = Object::cast_to<Button>(p_button);
 
 	Rect2 rect = button->get_screen_rect();
+	change_type->set_item_disabled(change_type->get_item_index(Variant::VARIANT_MAX), p_index < 0);
 	change_type->reset_size();
 	change_type->set_position(rect.get_end() - Vector2(change_type->get_contents_minimum_size().x, 0));
 	change_type->popup();