### 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_tree->clear();
	TreeItem *root = import_items_tree->create_item();
	if (base_theme.is_null()) {
		return;
	}
	String filter_text = import_items_filter->get_text();
	List<StringName> types;
	List<StringName> names;
	List<StringName> filtered_names;
	base_theme->get_type_list(&types);
	types.sort_custom<StringName::AlphCompare>();
	int color_amount = 0;
	int constant_amount = 0;
	int font_amount = 0;
	int font_size_amount = 0;
	int icon_amount = 0;
	int stylebox_amount = 0;
	tree_color_items.clear();
	tree_constant_items.clear();
	tree_font_items.clear();
	tree_font_size_items.clear();
	tree_icon_items.clear();
	tree_stylebox_items.clear();
	for (const StringName &E : types) {
		String type_name = (String)E;
		Ref<Texture2D> type_icon;
		if (E == "") {
			type_icon = get_theme_icon(SNAME("NodeDisabled"), SNAME("EditorIcons"));