### 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
ERR_FAIL_COND(p_event.is_null());
	Ref<InputEventKey> k = p_event;
	if ((k.is_valid() && k->is_pressed() && !k->is_echo()) || Object::cast_to<InputEventShortcut>(*p_event)) {
		EditorPlugin *old_editor = editor_plugin_screen;
		if (ED_IS_SHORTCUT("editor/next_tab", p_event)) {
			int next_tab = editor_data.get_edited_scene() + 1;
			next_tab %= editor_data.get_edited_scene_count();
			_scene_tab_changed(next_tab);
		}
		if (ED_IS_SHORTCUT("editor/prev_tab", p_event)) {
			int next_tab = editor_data.get_edited_scene() - 1;
			next_tab = next_tab >= 0 ? next_tab : editor_data.get_edited_scene_count() - 1;
			_scene_tab_changed(next_tab);
		}
		if (ED_IS_SHORTCUT("editor/filter_files", p_event)) {
			FileSystemDock::get_singleton()->focus_on_filter();
		}
		if (ED_IS_SHORTCUT("editor/editor_2d", p_event)) {
			editor_select(EDITOR_2D);
		} else if (ED_IS_SHORTCUT("editor/editor_3d", p_event)) {
			editor_select(EDITOR_3D);
		} else if (ED_IS_SHORTCUT("editor/editor_script", p_event)) {
			editor_select(EDITOR_SCRIPT);
		} else if (ED_IS_SHORTCUT("editor/editor_help", p_event)) {
			emit_signal(SNAME("request_help_search"), "");
		} else if (ED_IS_SHORTCUT("editor/editor_assetlib", p_event) && AssetLibraryEditorPlugin::is_available()) {
			editor_select(EDITOR_ASSETLIB);
		} else if (ED_IS_SHORTCUT("editor/editor_next", p_event)) {
			_editor_select_next();