fix crash when adding a repo without a fingerprint via URL/QR Code

src/org/fdroid/fdroid/views/fragments/RepoListFragment.java

@@ -364,6 +364,7 @@ public class RepoListFragment extends ListFragment
             final Button addButton = alrt.getButton(DialogInterface.BUTTON_POSITIVE);
             alrt.setTitle(R.string.repo_exists);
             overwriteMessage.setVisibility(View.VISIBLE);
+            if (newFingerprint != null)
                 newFingerprint = newFingerprint.toUpperCase(Locale.ENGLISH);
             if (repo.fingerprint == null && newFingerprint != null) {
                 // we're upgrading from unsigned to signed repo