### 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)

ERR_READ_THREAD_GUARD_V(false);
	return gui.dragging;
}
bool Viewport::gui_is_drag_successful() const {
	ERR_READ_THREAD_GUARD_V(false);
	return gui.drag_successful;
}
void Viewport::set_input_as_handled() {
	ERR_MAIN_THREAD_GUARD;
	if (!handle_input_locally) {
		ERR_FAIL_COND(!is_inside_tree());
		Viewport *vp = this;
		while (true) {
			if (Object::cast_to<Window>(vp)) {
				break;
			}
			if (!vp->get_parent()) {
				break;
			}
			vp = vp->get_parent()->get_viewport();
		}
		if (vp != this) {
			vp->set_input_as_handled();
			return;
		}
	}
	local_input_handled = true;
}
bool Viewport::is_input_handled() const {