### 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)

return;
	}
	if (gui.subwindow_focused) {
		if (gui.subwindow_focused == p_window) {
			return; 
		}
		gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_OUT);
		gui.subwindow_drag = SUB_WINDOW_DRAG_DISABLED;
	} else {
		Window *this_window = Object::cast_to<Window>(this);
		if (this_window) {
			this_window->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_OUT);
		}
	}
	Window *old_focus = gui.subwindow_focused;
	gui.subwindow_focused = p_window;
	gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_IN);
	{ 
		SubWindow sw = gui.sub_windows[index];
		gui.sub_windows.remove_at(index);
		gui.sub_windows.push_back(sw);
		index = gui.sub_windows.size() - 1;
		_sub_window_update_order();
	}
	if (old_focus) {
		_sub_window_update(old_focus);
	}
	_sub_window_update(p_window);
}
void Viewport::_sub_window_remove(Window *p_window) {