### Godot version

4.1.1

### System information

linux

### Issue description

[simplescreenrecorder-2023-08-30_15.01.51.webm](https://github.com/godotengine/godot-proposals/assets/71937306/537d92b9-4f98-4fe6-adf4-71297be0471a)

I guess this wasn't the case before since I could save a layout named "2 monitors".

### Steps to reproduce

press space

### Minimal reproduction project

none
ERR_FAIL_INDEX_MSG(p_tab, preview_tabs_content->get_child_count(), "Attempting to open a preview tab that doesn't exist.");
	for (int i = 0; i < preview_tabs_content->get_child_count(); i++) {
		Control *c = Object::cast_to<Control>(preview_tabs_content->get_child(i));
		if (!c) {
			continue;
		}
		c->set_visible(i == p_tab);
	}
}
void ThemeEditor::_remove_preview_tab(int p_tab) {
	ERR_FAIL_INDEX_MSG(p_tab, preview_tabs_content->get_child_count(), "Attempting to remove a preview tab that doesn't exist.");
	ThemeEditorPreview *preview_tab = Object::cast_to<ThemeEditorPreview>(preview_tabs_content->get_child(p_tab));
	ERR_FAIL_COND_MSG(Object::cast_to<DefaultThemeEditorPreview>(preview_tab), "Attemptying to remove the default preview tab.");
	if (preview_tab) {
		preview_tab->disconnect("control_picked", callable_mp(this, &ThemeEditor::_preview_control_picked));
		if (preview_tab->is_connected("scene_invalidated", callable_mp(this, &ThemeEditor::_remove_preview_tab_invalid))) {
			preview_tab->disconnect("scene_invalidated", callable_mp(this, &ThemeEditor::_remove_preview_tab_invalid));
		}
		if (preview_tab->is_connected("scene_reloaded", callable_mp(this, &ThemeEditor::_update_preview_tab))) {
			preview_tab->disconnect("scene_reloaded", callable_mp(this, &ThemeEditor::_update_preview_tab));
		}
		preview_tabs_content->remove_child(preview_tab);
		preview_tabs->remove_tab(p_tab);
		_change_preview_tab(preview_tabs->get_current_tab());
	}
}
void ThemeEditor::_remove_preview_tab_invalid(Node *p_tab_control) {