### 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)

diff.y = MAX(diff.y, -limit.y);
						r.size += diff;
					} break;
					default: {
					}
				}
				Size2i max_size = gui.currently_dragged_subwindow->get_max_size();
				if ((max_size.x > 0 || max_size.y > 0) && (max_size.x >= min_size.x && max_size.y >= min_size.y)) {
					max_size.x = MAX(max_size.x, 1);
					max_size.y = MAX(max_size.y, 1);
					if (r.size.x > max_size.x) {
						r.size.x = max_size.x;
					}
					if (r.size.y > max_size.y) {
						r.size.y = max_size.y;
					}
				}
				gui.currently_dragged_subwindow->_rect_changed_callback(r);
			}
			if (gui.currently_dragged_subwindow) { 
				_sub_window_update(gui.currently_dragged_subwindow);
			}
		}
		return true; 
	}
	Ref<InputEventMouseButton> mb = p_event;
	
	if (mb.is_valid() && mb->is_pressed() && mb->get_button_index() == MouseButton::LEFT) {
		Window *click_on_window = nullptr;
		for (int i = gui.sub_windows.size() - 1; i >= 0; i--) {