This happen when one attempt to create a new `GLib.Task' object.

    var task = yield new GLib.Task (null, null);

The signature should not be async and the expected use case is the following:

   var task = new GLib.Task (null, null, (obj, result) => {
       var bool_result = result.propagate_boolean ();
   });

   task.return_boolean (true);