prevent AppDetailsHeaderFragment crash in startProgress()

The real solution would involve figuring out where to handle this in the
right spot in the lifecycle.  Since AppDetails is being totally replaced,
this is just to stop the crashing.

closes #802

app/src/main/java/org/fdroid/fdroid/AppDetails.java

@@ -1525,9 +1525,11 @@ public class AppDetails extends AppCompatActivity {
 
         public void startProgress(boolean allowCancel) {
             cancelButton.setVisibility(allowCancel ? View.VISIBLE : View.GONE);
+            if (isAdded()) {
                 showIndeterminateProgress(getString(R.string.download_pending));
                 updateViews();
             }
+        }
 
         private void showIndeterminateProgress(String message) {
             setProgressVisible(true);