Fix null pointer exception

app/src/main/java/org/tasks/notifications/NotificationManager.java

@@ -163,11 +163,13 @@ public class NotificationManager {
 
             if (existingNotifications.size() == 1) {
                 org.tasks.notifications.Notification notification = existingNotifications.get(0);
-                NotificationCompat.Builder builder = getTaskNotification(notification)
+                NotificationCompat.Builder builder = getTaskNotification(notification);
-                        .setGroup(GROUP_KEY)
+                if (builder != null) {
+                    builder.setGroup(GROUP_KEY)
                             .setGroupAlertBehavior(NotificationCompat.GROUP_ALERT_SUMMARY);
                     notify(notification.taskId, builder, false, false, false);
                 }
+            }
 
             if (atLeastNougat() && newNotifications.size() == 1) {
                 org.tasks.notifications.Notification notification = newNotifications.get(0);