Clear parse worker cache when new book is opened

src/calibre/gui2/tweak_book/boss.py

@@ -97,6 +97,7 @@ class Boss(QObject):
             return error_dialog(self.gui, _('Failed to open book'),
                     _('Failed to open book, click Show details for more information.'),
                                 det_msg=job.traceback, show=True)
+        parse_worker.clear()
         container = job.result
         set_current_container(container)
         self.current_metadata = self.gui.current_metadata = container.mi

src/calibre/gui2/tweak_book/preview.py

@@ -117,6 +117,9 @@ class ParseWorker(Thread):
     def get_data(self, name):
         return getattr(self.parse_items.get(name, None), 'parsed_data', None)
 
+    def clear(self):
+        self.parse_items.clear()
+
 parse_worker = ParseWorker()
 # }}}