### 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
EditorProgress save("save", TTR("Saving Scene"), 4);
	if (editor_data.get_edited_scene_root() != nullptr) {
		save.step(TTR("Analyzing"), 0);
		int c2d = 0;
		int c3d = 0;
		_find_node_types(editor_data.get_edited_scene_root(), c2d, c3d);
		save.step(TTR("Creating Thumbnail"), 1);
		
		Ref<Image> img;
		
		
		
		if (c3d == 0 && c2d == 0) {
			img.instantiate();
			img->initialize_data(1, 1, false, Image::FORMAT_RGB8);
		} else if (c3d < c2d) {
			Ref<ViewportTexture> viewport_texture = scene_root->get_texture();
			if (viewport_texture->get_width() > 0 && viewport_texture->get_height() > 0) {
				img = viewport_texture->get_image();