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

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

@@ -691,19 +691,23 @@ public class NotificationsFragment extends SFragment implements
     private void saveNewestNotificationId(List<Notification> notifications) {
 
         AccountEntity account = accountManager.getActiveAccount();
-        BigInteger lastNoti = new BigInteger(account.getLastNotificationId());
+        if(account != null) {
+            BigInteger lastNoti = new BigInteger(account.getLastNotificationId());
 
-        for (Notification noti : notifications) {
-            BigInteger a = new BigInteger(noti.getId());
-            if (isBiggerThan(a, lastNoti)) {
-                lastNoti = a;
+            for (Notification noti : notifications) {
+                BigInteger a = new BigInteger(noti.getId());
+                if (isBiggerThan(a, lastNoti)) {
+                    lastNoti = a;
+                }
             }
-        }
-
-        Log.d(TAG, "saving newest noti id: " + lastNoti);
 
-        account.setLastNotificationId(lastNoti.toString());
-        accountManager.saveAccount(account);
+            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) {