Plugin Notification Ballon not visible.

Hi,

i'am writing a Plugin for PhpStorm that should display Notification Ballons. But my Problem is the Notifications are only in the Log.

here are the relevant code:

public class Poll implements StartupActivity {

    private NotificationGroup pollNotifications;

    @Override
    public void runActivity(@NotNull Project project) {

        pollNotifications = new NotificationGroup("Push Notifications", NotificationDisplayType.STICKY_BALLOON, true);

        // NotificationListener is a Thread
        NotificationListener client = new NotificationListener(remote, userEmail, new ReceiveHandler() {

            @Override
            public void receive(NotificationListener listener, String data) {
                ...
                displayNotification(title, content, t);
            }

        }
        client.start();

    }

    synchronized private void displayNotification(String title, String message, NotificationType type) {
        Notifications.Bus.notify(pollNotifications.createNotification(title, message, type, null));
    }

}

irrelevant try catch IOException is removed.