Check activity in callback

app/src/main/java/org/traccar/manager/StartFragment.java

@@ -87,12 +87,14 @@ public class StartFragment extends Fragment implements View.OnClickListener {
 
             @Override
             protected void onPostExecute(Boolean result) {
+                if (getActivity() != null) {
                     if (result) {
                         onSuccess();
                     } else {
                         onError();
                     }
                 }
+            }
 
         }.execute(serverField.getText().toString());
     }