### 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_tab_move_left->set_flat(true);
	if (gui_base->is_layout_rtl()) {
		dock_tab_move_left->set_icon(theme->get_icon(SNAME("Forward"), SNAME("EditorIcons")));
	} else {
		dock_tab_move_left->set_icon(theme->get_icon(SNAME("Back"), SNAME("EditorIcons")));
	}
	dock_tab_move_left->set_focus_mode(Control::FOCUS_NONE);
	dock_tab_move_left->connect("pressed", callable_mp(this, &EditorNode::_dock_move_left));
	dock_hb->add_child(dock_tab_move_left);
	Label *dock_label = memnew(Label);
	dock_label->set_text(TTR("Dock Position"));
	dock_label->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	dock_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
	dock_hb->add_child(dock_label);
	dock_tab_move_right = memnew(Button);
	dock_tab_move_right->set_flat(true);
	if (gui_base->is_layout_rtl()) {
		dock_tab_move_right->set_icon(theme->get_icon(SNAME("Back"), SNAME("EditorIcons")));
	} else {
		dock_tab_move_right->set_icon(theme->get_icon(SNAME("Forward"), SNAME("EditorIcons")));
	}
	dock_tab_move_right->set_focus_mode(Control::FOCUS_NONE);
	dock_tab_move_right->connect("pressed", callable_mp(this, &EditorNode::_dock_move_right));
	dock_hb->add_child(dock_tab_move_right);
	dock_vb->add_child(dock_hb);
	dock_select = memnew(Control);
	dock_select->set_custom_minimum_size(Size2(128, 64) * EDSCALE);
	dock_select->connect("gui_input", callable_mp(this, &EditorNode::_dock_select_input));
	dock_select->connect("draw", callable_mp(this, &EditorNode::_dock_select_draw));
	dock_select->connect("mouse_exited", callable_mp(this, &EditorNode::_dock_popup_exit));