The link for the parks file is not working: https://th-apex-soap-service.herokuapp.com/assets/parks.xml

I created the class - which works when testing it through Debug > Open Execute Anonymous Window , but when verifying this step I get this error:

Step not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.EmailException: SendEmail failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Either the plain text body or html body must be supplied.: [] 
Note: you may run into errors if you've skipped previous steps.

I created the class - which works when testing it through Debug > Open Execute Anonymous Window , but when verifying this step I get this error:

Step not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.EmailException: SendEmail failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Either the plain text body or html body must be supplied.: [] 
Note: you may run into errors if you've skipped previous steps.