### Godot version

4.1 rc3

### System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 30.0.15.1403) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

### Issue description

This™
![image](https://github.com/godotengine/godot/assets/2223172/cb33110f-a5b0-400a-951f-3fa425f05714)

It can still happen, but it's more difficult to trigger.

### Steps to reproduce

1. Create a new scene with a Control node
2. Add Theme to the Control
3. Open Theme Editor
4. Clear the `theme` property in Control
5. Save, with Theme Editor opened
6. Restart

### Minimal reproduction project

N/A
Ref<InputEventMouseButton> mb = p_input;
	if (mb.is_valid()) {
		if (scene_tabs->get_hovered_tab() >= 0) {
			if (mb->get_button_index() == MouseButton::MIDDLE && mb->is_pressed()) {
				_scene_tab_closed(scene_tabs->get_hovered_tab());
			}
		} else {
			if (mb->get_button_index() == MouseButton::LEFT && mb->is_double_click()) {
				_menu_option_confirm(FILE_NEW_SCENE, true);
			}
		}
		if (mb->get_button_index() == MouseButton::RIGHT && mb->is_pressed()) {
			
			scene_tabs_context_menu->clear();
			scene_tabs_context_menu->reset_size();
			scene_tabs_context_menu->add_shortcut(ED_GET_SHORTCUT("editor/new_scene"), FILE_NEW_SCENE);
			if (scene_tabs->get_hovered_tab() >= 0) {
				scene_tabs_context_menu->add_shortcut(ED_GET_SHORTCUT("editor/save_scene"), FILE_SAVE_SCENE);
				scene_tabs_context_menu->add_shortcut(ED_GET_SHORTCUT("editor/save_scene_as"), FILE_SAVE_AS_SCENE);
			}
			scene_tabs_context_menu->add_shortcut(ED_GET_SHORTCUT("editor/save_all_scenes"), FILE_SAVE_ALL_SCENES);
			if (scene_tabs->get_hovered_tab() >= 0) {
				scene_tabs_context_menu->add_separator();
				scene_tabs_context_menu->add_item(TTR("Show in FileSystem"), FILE_SHOW_IN_FILESYSTEM);
				scene_tabs_context_menu->add_item(TTR("Play This Scene"), FILE_RUN_SCENE);
				scene_tabs_context_menu->add_separator();
				scene_tabs_context_menu->add_shortcut(ED_GET_SHORTCUT("editor/close_scene"), FILE_CLOSE);
				scene_tabs_context_menu->set_item_text(scene_tabs_context_menu->get_item_index(FILE_CLOSE), TTR("Close Tab"));