make sure the dialog is still shown before dismissing

src/main/java/org/ligi/passandroid/ui/UnzipPassDialog.java

@@ -60,9 +60,12 @@ public class UnzipPassDialog {
                                 activity.runOnUiThread(new Runnable() {
                                     @Override
                                     public void run() {
+                                        if (myProgress.isShowing()) {
                                             myProgress.dismiss();
+                                            // TODO - check if it would be good to inform the user in another way
                                             DisplayError(activity, activity.getString(R.string.invalid_passbook_title), reason);
                                         }
+                                    }
                                 });
                             }
                         }