prevent crash when using back button after screen rotate

To reproduce the crash:
0. click a fdroidrepo:// URI to bring up the "app repo" dialog
1. rotate the device
2. click back to make the keyboard go away
3. click back to make the dialog go away
4. click back on Manage Repos screen
5. boom!

src/org/fdroid/fdroid/ManageRepo.java

@@ -59,9 +59,10 @@ public class ManageRepo extends FragmentActivity {
         ActionBarCompat.create(this).setDisplayHomeAsUpEnabled(true);
     }
 
+    @Override
     public void finish() {
         Intent ret = new Intent();
-        if (listFragment.hasChanged()) {
+        if (listFragment != null && listFragment.hasChanged()) {
             Log.i("FDroid", "Repo details have changed, prompting for update.");
             ret.putExtra(REQUEST_UPDATE, true);
         }