### 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
_find_and_save_edited_subresources(scene, processed, flags);
	for (int i = 0; i < scene->get_child_count(); i++) {
		Node *n = scene->get_child(i);
		if (n->get_owner() != editor_data.get_edited_scene_root()) {
			continue;
		}
		_save_edited_subresources(n, processed, flags);
	}
}
void EditorNode::_find_node_types(Node *p_node, int &count_2d, int &count_3d) {
	if (p_node->is_class("Viewport") || (p_node != editor_data.get_edited_scene_root() && p_node->get_owner() != editor_data.get_edited_scene_root())) {
		return;
	}
	if (p_node->is_class("CanvasItem")) {
		count_2d++;
	} else if (p_node->is_class("Node3D")) {
		count_3d++;
	}
	for (int i = 0; i < p_node->get_child_count(); i++) {
		_find_node_types(p_node->get_child(i), count_2d, count_3d);
	}
}
void EditorNode::_save_scene_with_preview(String p_file, int p_idx) {