QT: Fixed close button on file selection dialog.

xtt/lib/cow/qt/cow_wow_qt.cqt

@@ -521,13 +521,13 @@ void CoWowQt::CreateFileSelDia(const char* title, void* parent_ctx,
   QString filename = QFileDialog::getOpenFileName(
       object->parent_wid, fl(title), fl(folder), filter);
 
-  if (!filename.isNull()) {
+  if (!filename.isEmpty()) {
     strcpy(fname, qPrintable(filename));
-  }
 
     if (file_selected_cb) {
       (file_selected_cb)(parent_ctx, fname, file_type);
     }
+  }
 }
 
 CoWowTimer* CoWowQt::timer_new()