Fix #798441 (Download Metadata hangs Calibre)

src/calibre/gui2/metadata/single.py

@@ -388,6 +388,10 @@ class MetadataSingleDialogBase(ResizableDialog):
 
     def apply_changes(self):
         self.changed.add(self.book_id)
+        if self.db is None:
+            # break_cycles has already been called, don't know why this should
+            # happen but a user reported it
+            return True
         for widget in self.basic_metadata_widgets:
             try:
                 if not widget.commit(self.db, self.book_id):