Dont raise a KeyError in get_item_id() when the field does not exist

src/calibre/db/cache.py

@@ -810,7 +810,10 @@ class Cache:
     @read_api
     def get_item_id(self, field, item_name):
         ' Return the item id for item_name (case-insensitive) '
+        try:
             rmap = {icu_lower(v) if isinstance(v, str) else v:k for k, v in iteritems(self.fields[field].table.id_map)}
+        except KeyError:
+            rmap = {}
         return rmap.get(icu_lower(item_name) if isinstance(item_name, str) else item_name, None)
 
     @read_api