### 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
EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::INT, "interface/inspector/default_color_picker_mode", PROPERTY_HINT_ENUM, "RGB,HSV,RAW,OKHSL", PROPERTY_USAGE_DEFAULT));
	EDITOR_DEF("interface/inspector/default_color_picker_shape", (int32_t)ColorPicker::SHAPE_OKHSL_CIRCLE);
	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::INT, "interface/inspector/default_color_picker_shape", PROPERTY_HINT_ENUM, "HSV Rectangle,HSV Rectangle Wheel,VHS Circle,OKHSL Circle", PROPERTY_USAGE_DEFAULT));
	ED_SHORTCUT("canvas_item_editor/pan_view", TTR("Pan View"), Key::SPACE);
	const Vector<String> textfile_ext = ((String)(EDITOR_GET("docks/filesystem/textfile_extensions"))).split(",", false);
	for (const String &E : textfile_ext) {
		textfile_extensions.insert(E);
	}
	theme_base = memnew(Control);
	add_child(theme_base);
	theme_base->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT);
	gui_base = memnew(Panel);
	theme_base->add_child(gui_base);
	gui_base->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT);
	theme_base->set_theme(theme);
	gui_base->set_theme(theme);
	gui_base->add_theme_style_override("panel", gui_base->get_theme_stylebox(SNAME("Background"), SNAME("EditorStyles")));
	resource_preview = memnew(EditorResourcePreview);
	add_child(resource_preview);
	progress_dialog = memnew(ProgressDialog);
	progress_dialog->set_unparent_when_invisible(true);
	
	gui_base->set_anchor(SIDE_RIGHT, Control::ANCHOR_END);
	gui_base->set_anchor(SIDE_BOTTOM, Control::ANCHOR_END);
	gui_base->set_end(Point2(0, 0));
	main_vbox = memnew(VBoxContainer);
	gui_base->add_child(main_vbox);
	main_vbox->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT, Control::PRESET_MODE_MINSIZE, gui_base->get_theme_constant(SNAME("window_border_margin"), SNAME("Editor")));
	main_vbox->add_theme_constant_override("separation", gui_base->get_theme_constant(SNAME("top_bar_separation"), SNAME("Editor")));
	title_bar = memnew(EditorTitleBar);