### 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)

ERR_FAIL_COND(inspector_plugin_count == MAX_PLUGINS);
	int idx = -1;
	for (int i = 0; i < inspector_plugin_count; i++) {
		if (inspector_plugins[i] == p_plugin) {
			idx = i;
			break;
		}
	}
	ERR_FAIL_COND_MSG(idx == -1, "Trying to remove nonexistent inspector plugin.");
	for (int i = idx; i < inspector_plugin_count - 1; i++) {
		inspector_plugins[i] = inspector_plugins[i + 1];
	}
	inspector_plugins[inspector_plugin_count - 1] = Ref<EditorInspectorPlugin>();
	inspector_plugin_count--;
}
void EditorInspector::cleanup_plugins() {
	for (int i = 0; i < inspector_plugin_count; i++) {
		inspector_plugins[i].unref();
	}
	inspector_plugin_count = 0;
}
Button *EditorInspector::create_inspector_action_button(const String &p_text) {