Correct issue with exception on database change in Gramplets

Fixes #10147

gramps/gen/plug/_gramplet.py

@@ -399,7 +399,6 @@ class Gramplet:
         self.update()
 
     def _no_db(self):
-        if self.dbstate.db.is_open():
         self.disconnect_all()  # clear the old signals
 
     def get_option_widget(self, label):

gramps/gen/utils/callback.py

@@ -311,8 +311,7 @@ class Callback:
             for key in keymap:
                 self.__callback_map[signal_name].remove(key)
             self.__callback_map[signal_name] = None
-        self.__callback_map = None
-        del self.__callback_map
+        self.__callback_map = {}
 
     def emit(self, signal_name, args=tuple()):
         """