When the client app does not register to the ParsePush.PushNotificationReceived event, when a push notification is received, the event is fired, but since no-one had registred, it is null and a NullReferenceException is thrown on this line: PushNotificationReceived(ParseInstallation.CurrentInstallation, args);
The code should read something like this:
var handler = PushNotificationReceived;
if (handler != null)
{
handler(ParseInstallation.CurrentInstallation, args);
}