### 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)

gui.key_event_accepted = true;
	if (is_inside_tree()) {
		set_input_as_handled();
	}
}
void Viewport::_drop_mouse_focus() {
	Control *c = gui.mouse_focus;
	BitField<MouseButtonMask> mask = gui.mouse_focus_mask;
	gui.mouse_focus = nullptr;
	gui.forced_mouse_focus = false;
	gui.mouse_focus_mask.clear();
	for (int i = 0; i < 3; i++) {
		if ((int)mask & (1 << i)) {
			Ref<InputEventMouseButton> mb;
			mb.instantiate();
			mb->set_position(c->get_local_mouse_position());
			mb->set_global_position(c->get_local_mouse_position());
			mb->set_button_index(MouseButton(i + 1));
			mb->set_pressed(false);
			mb->set_device(InputEvent::DEVICE_ID_INTERNAL);
			c->_call_gui_input(mb);
		}
	}
}
void Viewport::_drop_physics_mouseover(bool p_paused_only) {