### 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
right_hsplit->add_child(right_l_vsplit);
	dock_slot[DOCK_SLOT_RIGHT_UL] = memnew(TabContainer);
	right_l_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_UL]);
	dock_slot[DOCK_SLOT_RIGHT_BL] = memnew(TabContainer);
	right_l_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_BL]);
	right_r_vsplit = memnew(VSplitContainer);
	right_hsplit->add_child(right_r_vsplit);
	dock_slot[DOCK_SLOT_RIGHT_UR] = memnew(TabContainer);
	right_r_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_UR]);
	dock_slot[DOCK_SLOT_RIGHT_BR] = memnew(TabContainer);
	right_r_vsplit->add_child(dock_slot[DOCK_SLOT_RIGHT_BR]);
	
	vsplits.push_back(left_l_vsplit);
	vsplits.push_back(left_r_vsplit);
	vsplits.push_back(right_l_vsplit);
	vsplits.push_back(right_r_vsplit);
	hsplits.push_back(left_l_hsplit);
	hsplits.push_back(left_r_hsplit);
	hsplits.push_back(main_hsplit);
	hsplits.push_back(right_hsplit);
	for (int i = 0; i < vsplits.size(); i++) {
		vsplits[i]->connect("dragged", callable_mp(this, &EditorNode::_dock_split_dragged));
		hsplits[i]->connect("dragged", callable_mp(this, &EditorNode::_dock_split_dragged));
	}
	dock_select_popup = memnew(PopupPanel);
	gui_base->add_child(dock_select_popup);
	VBoxContainer *dock_vb = memnew(VBoxContainer);
	dock_select_popup->add_child(dock_vb);
	HBoxContainer *dock_hb = memnew(HBoxContainer);
	dock_tab_move_left = memnew(Button);