### 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)

case NOTIFICATION_VP_MOUSE_ENTER: {
			gui.mouse_in_viewport = true;
		} break;
		case NOTIFICATION_VP_MOUSE_EXIT: {
			gui.mouse_in_viewport = false;
			_drop_physics_mouseover();
			
			
			
		} break;
		case NOTIFICATION_WM_WINDOW_FOCUS_OUT: {
			_gui_cancel_tooltip();
			_drop_physics_mouseover();
			if (gui.mouse_focus && !gui.forced_mouse_focus) {
				_drop_mouse_focus();
			}
			
			
			
			
		} break;
		case NOTIFICATION_PREDELETE: {
			if (gui_parent) {
				gui_parent->gui.tooltip_popup = nullptr;
				gui_parent->gui.tooltip_label = nullptr;
			}
		} break;
	}
}
void Viewport::_process_picking() {