title: Issue in Displaying Notification with awesome_notifications in Flutter
body: <p>I am new to flutter.I have started to create a reminder app.I am trying to call a push notification on android alarm call back.I am using <strong>awesome_notifications</strong>. On call back I am calling <code>_ringAlarm()</code>.I have pasted it below.
  It says <code>D/NotificationSender( 5192): Notification created</code>, but nothing shows on screen.</p>
  <p>This exception was thrown while running:</p>
  <blockquote>
  <p>W/System.err( 5192): java.lang.IllegalArgumentException: Invalid
  notification (no valid small icon): Notification(channel=basic_channel
  shortcut=null contentView=null vibrate=null sound=null tick
  defaults=0x0 flags=0x11 color=0xff9d50dd vis=UNKNOWN(2))</p>
  </blockquote>
  <pre><code>Future _ringAlarm() async {
  
    AwesomeNotifications().initialize(
      'resource://drawable/logo.png',
      [
          NotificationChannel(
              channelKey: 'basic_channel',
              channelName: 'Basic notifications',
              channelDescription: 'Notification channel for basic tests',
              defaultColor: Color(0xFF9D50DD),
              ledColor: Colors.white
          )
      ]
    );
  
    AwesomeNotifications().isNotificationAllowed().then((isAllowed) {
      if (!isAllowed) {
        AwesomeNotifications().requestPermissionToSendNotifications();
      }
    });
  
    AwesomeNotifications().createNotification(
      content: NotificationContent(
          id: 10,
          channelKey: 'basic_channel',
          title: 'Simple Notification',
          body: 'Simple body'
      )
    );
  
  }
  </code></pre>