Template type is not string anymore

A template, used as a parameter for a “message:” in a notification, doesn’t result in a string anymore. This has obvioulsy to do with the new function in 0.118 “Native types support for templates”.

I can not find out, how to change the template in order to make it work again under the new template engine. My template creates a json format string from the trigger event data:

- service: notify.file_sensor
  data_template:
     message: '{"pic_url": "{{ trigger.event.data["data"]["snapshot_url"] }}"}'

Has anbody a hint for me, how to change the template?