Prevent crash

app/src/main/java/eu/faircode/email/BoundaryCallbackMessages.java

@@ -60,6 +60,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
                         @Override
                         public void run() {
                             model.clear();
+                            model = null;
                         }
                     });
             }
@@ -83,6 +84,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
     }
 
     private void load() {
+        if (model != null)
             executor.submit(new Runnable() {
                 @Override
                 public void run() {