Fix crash when undoing a thread hide while refreshing

Clover/app/src/main/java/org/floens/chan/core/presenter/ThreadPresenter.java

@@ -191,8 +191,10 @@ public class ThreadPresenter implements ChanLoader.ChanLoaderCallback, PostAdapt
     }
 
     public void refreshUI() {
+        if (chanLoader.getThread() != null) {
             showPosts();
         }
+    }
 
     @Override
     public Loadable getLoadable() {