### 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
dock_popup_selected_idx = p_which;
}
void EditorNode::_dock_move_left() {
	if (dock_popup_selected_idx < 0 || dock_popup_selected_idx >= DOCK_SLOT_MAX) {
		return;
	}
	Control *current_ctl = dock_slot[dock_popup_selected_idx]->get_tab_control(dock_slot[dock_popup_selected_idx]->get_current_tab());
	Control *prev_ctl = dock_slot[dock_popup_selected_idx]->get_tab_control(dock_slot[dock_popup_selected_idx]->get_current_tab() - 1);
	if (!current_ctl || !prev_ctl) {
		return;
	}
	dock_slot[dock_popup_selected_idx]->move_child(current_ctl, prev_ctl->get_index(false));
	dock_select->queue_redraw();
	_edit_current();
	_save_editor_layout();
}
void EditorNode::_dock_move_right() {
	Control *current_ctl = dock_slot[dock_popup_selected_idx]->get_tab_control(dock_slot[dock_popup_selected_idx]->get_current_tab());
	Control *next_ctl = dock_slot[dock_popup_selected_idx]->get_tab_control(dock_slot[dock_popup_selected_idx]->get_current_tab() + 1);
	if (!current_ctl || !next_ctl) {
		return;
	}
	dock_slot[dock_popup_selected_idx]->move_child(next_ctl, current_ctl->get_index(false));
	dock_select->queue_redraw();
	_edit_current();
	_save_editor_layout();
}
void EditorNode::_dock_select_draw() {