Clear boundary

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

@@ -773,6 +773,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
 
     void destroy(State state) {
         state.destroyed = true;
+        this.intf = null;
         Log.i("Boundary destroy");
 
         executor.submit(new Runnable() {

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

@@ -603,6 +603,7 @@ public class ViewModelMessages extends ViewModel {
                     @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
                     public void onDestroyed() {
                         boundary.destroy(state);
+                        boundary = null;
                         owner.getLifecycle().removeObserver(this);
                     }
                 });