### Godot version

v4.0 rc6

### System information

Windows 10

### Issue description

When using a ViewportTexture from a SubViewport on a TextureRect, it throws errors after running the scene once and then switching scenes in the editor.
![Pasted image 20230227212626](https://user-images.githubusercontent.com/44676282/221680047-984fa03e-33d1-4856-9a7f-1700b28423cf.png)


### Steps to reproduce

Run the canvas-scene once.
Switch to another scene in the editor.
Switch back to the canvas-scene

### Minimal reproduction project

[ViewportTextureError.zip](https://github.com/godotengine/godot/files/10844099/ViewportTextureError.zip)

if (p_window == nullptr) {
		
		if (gui.subwindow_focused) {
			gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_OUT);
			gui.subwindow_focused = nullptr;
			gui.subwindow_drag = SUB_WINDOW_DRAG_DISABLED;
		}
		Window *this_window = Object::cast_to<Window>(this);
		if (this_window) {
			this_window->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_IN);
		}
		return;
	}
	int index = _sub_window_find(p_window);
	ERR_FAIL_COND(index == -1);
	if (p_window->get_flag(Window::FLAG_NO_FOCUS)) {
		
		if (gui.subwindow_focused) {
			gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_OUT);
			gui.subwindow_focused = nullptr;
			gui.subwindow_drag = SUB_WINDOW_DRAG_DISABLED;
		}
		
		SubWindow sw = gui.sub_windows[index];
		gui.sub_windows.remove_at(index);
		gui.sub_windows.push_back(sw);
		_sub_window_update_order();