### 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
if (p_editing_owner) {
		const ObjectID id = p_editing_owner->get_instance_id();
		for (EditorPlugin *plugin : active_plugins[id]) {
			_plugin_over_edit(plugin, nullptr);
		}
		active_plugins.erase(id);
	} else {
		
		
		List<ObjectID> to_remove;
		for (KeyValue<ObjectID, HashSet<EditorPlugin *>> &kv : active_plugins) {
			if (!ObjectDB::get_instance(kv.key)) {
				to_remove.push_back(kv.key);
				for (EditorPlugin *plugin : kv.value) {
					_plugin_over_edit(plugin, nullptr);
				}
			}
		}
		for (const ObjectID &id : to_remove) {
			active_plugins.erase(id);
		}
	}
}
static bool overrides_external_editor(Object *p_object) {