### 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
Ref<InputEventMouse> me = p_input;
	if (me.is_valid()) {
		Vector2 point = me->get_position();
		int nrect = -1;
		for (int i = 0; i < DOCK_SLOT_MAX; i++) {
			if (dock_select_rect[i].has_point(point)) {
				nrect = i;
				break;
			}
		}
		if (nrect != dock_select_rect_over_idx) {
			dock_select->queue_redraw();
			dock_select_rect_over_idx = nrect;
		}
		if (nrect == -1) {