### 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)

TreeItem *icon_root = edit_items_tree->create_item(root);
			icon_root->set_metadata(0, Theme::DATA_TYPE_ICON);
			icon_root->set_icon(0, get_theme_icon(SNAME("ImageTexture"), SNAME("EditorIcons")));
			icon_root->set_text(0, TTR("Icons"));
			icon_root->add_button(0, get_theme_icon(SNAME("Clear"), SNAME("EditorIcons")), ITEMS_TREE_REMOVE_DATA_TYPE, false, TTR("Remove All Icon Items"));
			names.sort_custom<StringName::AlphCompare>();
			for (const StringName &E : names) {
				TreeItem *item = edit_items_tree->create_item(icon_root);
				item->set_text(0, E);
				item->add_button(0, get_theme_icon(SNAME("Edit"), SNAME("EditorIcons")), ITEMS_TREE_RENAME_ITEM, false, TTR("Rename Item"));
				item->add_button(0, get_theme_icon(SNAME("Remove"), SNAME("EditorIcons")), ITEMS_TREE_REMOVE_ITEM, false, TTR("Remove Item"));
			}
			has_any_items = true;
		}
	}
	{ 
		names.clear();
		edited_theme->get_stylebox_list(p_item_type, &names);
		if (names.size() > 0) {
			TreeItem *stylebox_root = edit_items_tree->create_item(root);
			stylebox_root->set_metadata(0, Theme::DATA_TYPE_STYLEBOX);
			stylebox_root->set_icon(0, get_theme_icon(SNAME("StyleBoxFlat"), SNAME("EditorIcons")));
			stylebox_root->set_text(0, TTR("Styleboxes"));
			stylebox_root->add_button(0, get_theme_icon(SNAME("Clear"), SNAME("EditorIcons")), ITEMS_TREE_REMOVE_DATA_TYPE, false, TTR("Remove All StyleBox Items"));
			names.sort_custom<StringName::AlphCompare>();
			for (const StringName &E : names) {
				TreeItem *item = edit_items_tree->create_item(stylebox_root);
				item->set_text(0, E);
				item->add_button(0, get_theme_icon(SNAME("Edit"), SNAME("EditorIcons")), ITEMS_TREE_RENAME_ITEM, false, TTR("Rename Item"));
				item->add_button(0, get_theme_icon(SNAME("Remove"), SNAME("EditorIcons")), ITEMS_TREE_REMOVE_ITEM, false, TTR("Remove Item"));