Fix running check book with no book opened gives an error message

src/calibre/gui2/tweak_book/boss.py

@@ -795,6 +795,8 @@ class Boss(QObject):
 
     @in_thread_job
     def check_requested(self, *args):
+        if current_container() is None:
+            return
         self.commit_all_editors_to_container()
         c = self.gui.check_book
         c.parent().show()