### 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
project_menu->add_child(vcs_actions_menu);
	project_menu->set_item_submenu(project_menu->get_item_index(VCS_MENU), "Version Control");
	add_editor_plugin(memnew(AudioBusesEditorPlugin(audio_bus_editor)));
	for (int i = 0; i < EditorPlugins::get_plugin_count(); i++) {
		add_editor_plugin(EditorPlugins::create(i));
	}
	for (const StringName &extension_class_name : GDExtensionEditorPlugins::get_extension_classes()) {
		add_extension_editor_plugin(extension_class_name);
	}
	GDExtensionEditorPlugins::editor_node_add_plugin = &EditorNode::add_extension_editor_plugin;
	GDExtensionEditorPlugins::editor_node_remove_plugin = &EditorNode::remove_extension_editor_plugin;
	for (int i = 0; i < plugin_init_callback_count; i++) {
		plugin_init_callbacks[i]();
	}
	resource_preview->add_preview_generator(Ref<EditorTexturePreviewPlugin>(memnew(EditorTexturePreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorImagePreviewPlugin>(memnew(EditorImagePreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorPackedScenePreviewPlugin>(memnew(EditorPackedScenePreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorMaterialPreviewPlugin>(memnew(EditorMaterialPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorScriptPreviewPlugin>(memnew(EditorScriptPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorAudioStreamPreviewPlugin>(memnew(EditorAudioStreamPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorMeshPreviewPlugin>(memnew(EditorMeshPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorBitmapPreviewPlugin>(memnew(EditorBitmapPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorFontPreviewPlugin>(memnew(EditorFontPreviewPlugin)));
	resource_preview->add_preview_generator(Ref<EditorGradientPreviewPlugin>(memnew(EditorGradientPreviewPlugin)));
	{
		Ref<StandardMaterial3DConversionPlugin> spatial_mat_convert;
		spatial_mat_convert.instantiate();
		resource_conversion_plugins.push_back(spatial_mat_convert);
		Ref<ORMMaterial3DConversionPlugin> orm_mat_convert;
		orm_mat_convert.instantiate();