### 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)

next = from->_get_focus_neighbor(SIDE_BOTTOM);
				}
			} else {
				if (p_event->is_action_pressed("ui_focus_next", true, true)) {
					next = from->find_next_valid_focus();
				}
				if (p_event->is_action_pressed("ui_focus_prev", true, true)) {
					next = from->find_prev_valid_focus();
				}
				if (p_event->is_action_pressed("ui_up", true, true)) {
					next = from->_get_focus_neighbor(SIDE_TOP);
				}
				if (p_event->is_action_pressed("ui_left", true, true)) {
					next = from->_get_focus_neighbor(SIDE_LEFT);
				}
				if (p_event->is_action_pressed("ui_right", true, true)) {
					next = from->_get_focus_neighbor(SIDE_RIGHT);
				}
				if (p_event->is_action_pressed("ui_down", true, true)) {
					next = from->_get_focus_neighbor(SIDE_BOTTOM);
				}
			}
			if (next) {
				next->grab_focus();
				set_input_as_handled();
			}
		}
	}
}
void Viewport::_perform_drop(Control *p_control, Point2 p_pos) {