### 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)

edit_theme_item_dialog->set_title(TTR("Add Stylebox Item"));
			break;
		case Theme::DATA_TYPE_MAX:
			break; 
	}
	edit_theme_item_old_vb->hide();
	theme_item_name->clear();
	edit_theme_item_dialog->popup_centered(Size2(380, 110) * EDSCALE);
	theme_item_name->grab_focus();
}
void ThemeItemEditorDialog::_open_rename_theme_item_dialog(Theme::DataType p_data_type, String p_item_name) {
	ERR_FAIL_INDEX_MSG(p_data_type, Theme::DATA_TYPE_MAX, "Theme item data type is out of bounds.");
	item_popup_mode = RENAME_THEME_ITEM;
	edit_item_data_type = p_data_type;
	edit_item_old_name = p_item_name;
	switch (edit_item_data_type) {
		case Theme::DATA_TYPE_COLOR:
			edit_theme_item_dialog->set_title(TTR("Rename Color Item"));
			break;
		case Theme::DATA_TYPE_CONSTANT:
			edit_theme_item_dialog->set_title(TTR("Rename Constant Item"));
			break;
		case Theme::DATA_TYPE_FONT:
			edit_theme_item_dialog->set_title(TTR("Rename Font Item"));
			break;
		case Theme::DATA_TYPE_FONT_SIZE:
			edit_theme_item_dialog->set_title(TTR("Rename Font Size Item"));
			break;
		case Theme::DATA_TYPE_ICON:
			edit_theme_item_dialog->set_title(TTR("Rename Icon Item"));