Fix #3724 ("AttributeError" when canceling multiple book conversion)

src/calibre/gui2/main.py

@@ -1200,6 +1200,8 @@ class Main(MainWindow, Ui_MainWindow, DeviceGUI):
                 self.library_view.model().db, book_ids,
                 out_format=prefs['output_format'], args=(rows, previous,
                     self.book_converted))
+            if self.__bulk_queue is None:
+                return
             num = len(self.__bulk_queue.book_ids)
         else:
             jobs, changed, bad = convert_single_ebook(self,

src/calibre/gui2/tools.py

@@ -105,7 +105,7 @@ def convert_bulk_ebook(parent, queue, db, book_ids, out_format=None, args=[]):
 
     d = BulkConfig(parent, db, out_format)
     if d.exec_() != QDialog.Accepted:
-        return jobs, changed, bad
+        return None
 
     output_format = d.output_format
     user_recs = cPickle.loads(d.recommendations)