### 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)

if (p_visible) {
		button->show();
		EditorNode::get_singleton()->make_bottom_panel_item_visible(theme_editor);
	} else {
		if (theme_editor->is_visible_in_tree()) {
			EditorNode::get_singleton()->hide_bottom_panel();
		}
		button->hide();
	}
}
ThemeEditorPlugin::ThemeEditorPlugin() {
	theme_editor = memnew(ThemeEditor);
	theme_editor->set_custom_minimum_size(Size2(0, 200) * EDSCALE);
	button = EditorNode::get_singleton()->add_bottom_panel_item(TTR("Theme"), theme_editor);
	button->hide();
}