### 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
top_split->set_collapsed(true);
	VBoxContainer *srt = memnew(VBoxContainer);
	srt->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	top_split->add_child(srt);
	srt->add_theme_constant_override("separation", 0);
	tab_preview_panel = memnew(Panel);
	tab_preview_panel->set_size(Size2(100, 100) * EDSCALE);
	tab_preview_panel->hide();
	tab_preview_panel->set_self_modulate(Color(1, 1, 1, 0.7));
	gui_base->add_child(tab_preview_panel);
	tab_preview = memnew(TextureRect);
	tab_preview->set_stretch_mode(TextureRect::STRETCH_KEEP_ASPECT_CENTERED);
	tab_preview->set_size(Size2(96, 96) * EDSCALE);
	tab_preview->set_position(Point2(2, 2) * EDSCALE);
	tab_preview_panel->add_child(tab_preview);
	tabbar_panel = memnew(PanelContainer);
	tabbar_panel->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("tabbar_background"), SNAME("TabContainer")));
	srt->add_child(tabbar_panel);
	tabbar_container = memnew(HBoxContainer);
	tabbar_panel->add_child(tabbar_container);
	scene_tabs = memnew(TabBar);
	scene_tabs->set_select_with_rmb(true);
	scene_tabs->add_tab("unsaved");
	scene_tabs->set_tab_close_display_policy((TabBar::CloseButtonDisplayPolicy)EDITOR_GET("interface/scene_tabs/display_close_button").operator int());
	scene_tabs->add_theme_constant_override("icon_max_width", gui_base->get_theme_constant(SNAME("class_icon_size"), SNAME("Editor")));
	scene_tabs->set_max_tab_width(int(EDITOR_GET("interface/scene_tabs/maximum_width")) * EDSCALE);
	scene_tabs->set_drag_to_rearrange_enabled(true);
	scene_tabs->set_auto_translate(false);
	scene_tabs->connect("tab_changed", callable_mp(this, &EditorNode::_scene_tab_changed));
	scene_tabs->connect("tab_button_pressed", callable_mp(this, &EditorNode::_scene_tab_script_edited));