### 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_slot[DOCK_SLOT_LEFT_UR]->set_tab_title(dock_slot[DOCK_SLOT_LEFT_UR]->get_tab_idx_from_control(SceneTreeDock::get_singleton()), TTR("Scene"));
	
	dock_slot[DOCK_SLOT_LEFT_UR]->add_child(ImportDock::get_singleton());
	dock_slot[DOCK_SLOT_LEFT_UR]->set_tab_title(dock_slot[DOCK_SLOT_LEFT_UR]->get_tab_idx_from_control(ImportDock::get_singleton()), TTR("Import"));
	
	dock_slot[DOCK_SLOT_LEFT_BR]->add_child(FileSystemDock::get_singleton());
	dock_slot[DOCK_SLOT_LEFT_BR]->set_tab_title(dock_slot[DOCK_SLOT_LEFT_BR]->get_tab_idx_from_control(FileSystemDock::get_singleton()), TTR("FileSystem"));
	
	dock_slot[DOCK_SLOT_RIGHT_UL]->add_child(InspectorDock::get_singleton());
	dock_slot[DOCK_SLOT_RIGHT_UL]->set_tab_title(dock_slot[DOCK_SLOT_RIGHT_UL]->get_tab_idx_from_control(InspectorDock::get_singleton()), TTR("Inspector"));
	
	dock_slot[DOCK_SLOT_RIGHT_UL]->add_child(NodeDock::get_singleton());
	dock_slot[DOCK_SLOT_RIGHT_UL]->set_tab_title(dock_slot[DOCK_SLOT_RIGHT_UL]->get_tab_idx_from_control(NodeDock::get_singleton()), TTR("Node"));
	
	dock_slot[DOCK_SLOT_RIGHT_UL]->add_child(history_dock);
	dock_slot[DOCK_SLOT_RIGHT_UL]->set_tab_title(dock_slot[DOCK_SLOT_RIGHT_UL]->get_tab_idx_from_control(history_dock), TTR("History"));
	
	dock_slot[DOCK_SLOT_LEFT_UL]->hide();
	dock_slot[DOCK_SLOT_LEFT_BL]->hide();
	dock_slot[DOCK_SLOT_RIGHT_BL]->hide();
	dock_slot[DOCK_SLOT_RIGHT_UR]->hide();
	dock_slot[DOCK_SLOT_RIGHT_BR]->hide();
	left_l_vsplit->hide();
	right_r_vsplit->hide();
	
	left_r_hsplit->set_split_offset(270 * EDSCALE);
	main_hsplit->set_split_offset(-270 * EDSCALE);
	
	const String docks_section = "docks";
	default_layout.instantiate();