### 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)

if (!is_inside_tree() || is_input_disabled()) {
		return;
	}
	if (gui.subwindow_over) {
		gui.subwindow_over->_mouse_leave_viewport();
		gui.subwindow_over = nullptr;
	} else if (gui.mouse_over) {
		_drop_mouse_over();
	}
	notification(NOTIFICATION_VP_MOUSE_EXIT);
}
void Viewport::_drop_mouse_over() {
	_gui_cancel_tooltip();
	SubViewportContainer *c = Object::cast_to<SubViewportContainer>(gui.mouse_over);
	if (c) {
		for (int i = 0; i < c->get_child_count(); i++) {
			SubViewport *v = Object::cast_to<SubViewport>(c->get_child(i));
			if (!v) {
				continue;
			}
			v->_mouse_leave_viewport();
		}
	}
	if (gui.mouse_over->is_inside_tree()) {
		gui.mouse_over->notification(Control::NOTIFICATION_MOUSE_EXIT);
	}
	gui.mouse_over = nullptr;
}
void Viewport::push_input(const Ref<InputEvent> &p_event, bool p_local_coords) {