fix crash in notifications fragment, save account only when notificationid changed

app/src/main/java/com/keylesspalace/tusky/fragment/NotificationsFragment.java

@@ -691,6 +691,7 @@ public class NotificationsFragment extends SFragment implements
     private void saveNewestNotificationId(List<Notification> notifications) {
 
         AccountEntity account = accountManager.getActiveAccount();
+        if(account != null) {
             BigInteger lastNoti = new BigInteger(account.getLastNotificationId());
 
             for (Notification noti : notifications) {
@@ -700,11 +701,14 @@ public class NotificationsFragment extends SFragment implements
                 }
             }
 
-        Log.d(TAG, "saving newest noti id: " + lastNoti);
-
-        account.setLastNotificationId(lastNoti.toString());
+            String lastNotificationId = lastNoti.toString();
+            if(!account.getLastNotificationId().equals(lastNotificationId)) {
+                Log.d(TAG, "saving newest noti id: " + lastNotificationId);
+                account.setLastNotificationId(lastNotificationId);
                 accountManager.saveAccount(account);
             }
+        }
+    }
 
     private boolean isBiggerThan(BigInteger newId, BigInteger lastShownNotificationId) {
         return lastShownNotificationId.compareTo(newId) < 0;