### Godot version

v4.0.beta17.mono.official [c40020513]

### System information

Window 11, Forward+, GTX 1060

### Issue description

Adding two theme variations in a row result to a  crash.

https://user-images.githubusercontent.com/93253123/217037249-1ce1d56d-beb6-4435-83ad-9f8a07fe0e96.mp4



### Steps to reproduce

1. Create a project.
2. Create a theme.
3. Double click to go in the theme editor.
4. Click on manage items buttons.
5. In the add type field type a name to add a type.
6. type another name to add a second type.
7. Crash.

### Minimal reproduction project

[ThemeVariationCrash.zip](https://github.com/godotengine/godot/files/10665863/ThemeVariationCrash.zip)

import_items_filter->clear();
	selected_items.clear();
	total_selected_colors_label->hide();
	total_selected_constants_label->hide();
	total_selected_fonts_label->hide();
	total_selected_font_sizes_label->hide();
	total_selected_icons_label->hide();
	total_selected_styleboxes_label->hide();
	_update_items_tree();
}
bool ThemeItemImportTree::has_selected_items() const {
	return (selected_items.size() > 0);
}
void ThemeItemImportTree::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_ENTER_TREE:
		case NOTIFICATION_THEME_CHANGED: {
			select_icons_warning_icon->set_texture(get_theme_icon(SNAME("StatusWarning"), SNAME("EditorIcons")));
			select_icons_warning->add_theme_color_override("font_color", get_theme_color(SNAME("disabled_font_color"), SNAME("Editor")));
			import_items_filter->set_right_icon(get_theme_icon(SNAME("Search"), SNAME("EditorIcons")));
			
			import_collapse_types_button->set_icon(get_theme_icon(SNAME("CollapseTree"), SNAME("EditorIcons")));
			import_expand_types_button->set_icon(get_theme_icon(SNAME("ExpandTree"), SNAME("EditorIcons")));
			import_select_all_button->set_icon(get_theme_icon(SNAME("ThemeSelectAll"), SNAME("EditorIcons")));
			import_select_full_button->set_icon(get_theme_icon(SNAME("ThemeSelectFull"), SNAME("EditorIcons")));
			import_deselect_all_button->set_icon(get_theme_icon(SNAME("ThemeDeselectAll"), SNAME("EditorIcons")));
			
			select_colors_icon->set_texture(get_theme_icon(SNAME("Color"), SNAME("EditorIcons")));