checking valuelist in process_formdata for Select2TagsField before using it

flask_admin/form/fields.py

@@ -166,6 +166,7 @@ class Select2TagsField(fields.StringField):
         super(Select2TagsField, self).__init__(label, validators, **kwargs)
 
     def process_formdata(self, valuelist):
+        if valuelist:
             if self.save_as_list:
                 self.data = [self.coerce(v.strip()) for v in valuelist[0].split(',') if v.strip()]
             else: