Adding a NotebookPage without child crashes

[Fighter19]

Tested with version v47.1. (Current HEAD of development branch).

When adding a NotebookPage inside a Notebook, the application crashes.

(re.sonny.Workbench.Devel:5521): Gtk-CRITICAL **: 15:48:11.837: gtk_stack_add_named: assertion 'GTK_IS_WIDGET (child)' failed

Working:

using Gtk 4.0;

Notebook {
  NotebookPage {
    child: Label {
      label: _("Test");
    };
  }
}

Broken:

using Gtk 4.0;

Notebook {
  NotebookPage {

  }
}

This occurs, while writing normally. The blueprint file needs to be manually modified to prevent the application from crashing on load.