Fix possible null pointer exception.

It is weird anyway.

Change-Id: Ibde6e7dc7dc52c556ebc5e6e32ef939a316c5b1b

android/sdremote/src/org/libreoffice/impressremote/fragment/ComputersFragment.java

@@ -152,6 +152,10 @@ public class ComputersFragment extends SherlockListFragment implements ServiceCo
 
     @Override
     public void run() {
+        if (!isAdded()) {
+            return;
+        }
+
         if (isShowingProgressMessageRequired()) {
             showProgressMessage();
         }