### Godot version

v4.0.beta.custom_build [eaf306e0b]

### System information

Windows 10

### Issue description

Selecting a `Window` node in the Editor's SceneTree results in the message
```
editor/editor_data.cpp:1058 - Parameter "p_node" is null.
```
and the Inspector shows empty content instead of the Window's properties.

I would expect, that clicking on a node in the SceneTree lets me edit the properties of the node.

Bisecting shows that this behavior got introduced in #71770

### Steps to reproduce

1. Create a Window node
2. Click on the Window node in the Editor's SceneTree

### Minimal reproduction project

N/A
List<BaseButton *> btn_list;
	button_group->get_buttons(&btn_list);
	for (int i = 0; i < btn_list.size(); i++) {
		CheckBox *check = Object::cast_to<CheckBox>(btn_list[i]);
		check->set_icon(get_theme_icon(check->get_text(), SNAME("EditorIcons")));
	}
	label->add_theme_color_override("font_color", get_theme_color(SNAME("warning_color"), SNAME("Editor")));
}
void CanvasItemEditorViewport::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_THEME_CHANGED: {
			_update_theme();
		} break;
		case NOTIFICATION_ENTER_TREE: {
			_update_theme();
			connect("mouse_exited", callable_mp(this, &CanvasItemEditorViewport::_on_mouse_exit));
		} break;
		case NOTIFICATION_EXIT_TREE: {
			disconnect("mouse_exited", callable_mp(this, &CanvasItemEditorViewport::_on_mouse_exit));
		} break;
	}
}
CanvasItemEditorViewport::CanvasItemEditorViewport(CanvasItemEditor *p_canvas_item_editor) {