Steps to reproduce:

1. Create an HTTP Agent item
2. Change data type to Text
3. Add https://www.zabbix.com/documentation/current/en/manual/api/reference/action/get as the URL (on line help from Zabbix)
4. Test the item -> ok
5. Add a preprocessing step "XML to JSON" -> Error
6. ...

Result:
Expected:
The XML converted to JSON or some restrictions specified in the online help