### 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)

subwindow_canvas = RID();
	}
	if (gui.currently_dragged_subwindow == p_window) {
		gui.subwindow_drag = SUB_WINDOW_DRAG_DISABLED;
		gui.currently_dragged_subwindow = nullptr;
	}
	if (gui.subwindow_focused == p_window) {
		Window *new_focused_window;
		Window *parent_visible = p_window->get_parent_visible_window();
		gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_OUT);
		if (parent_visible) {
			new_focused_window = parent_visible;
		} else {
			new_focused_window = Object::cast_to<Window>(this);
		}
		if (new_focused_window) {
			int new_focused_index = _sub_window_find(new_focused_window);
			if (new_focused_index != -1) {
				gui.subwindow_focused = new_focused_window;
			} else {
				gui.subwindow_focused = nullptr;
			}
			new_focused_window->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_IN);
		} else {
			gui.subwindow_focused = nullptr;
		}
	}
	RenderingServer::get_singleton()->viewport_set_parent_viewport(p_window->viewport, p_window->parent ? p_window->parent->viewport : RID());
}
int Viewport::_sub_window_find(Window *p_window) const {