SendRazorEmail returns Object reference not set to Instance of Object

Questions
1

After submitting form, user is navigated to external site then returns for an email to be sent using SendRazorEmail with a template.

q1

Then create the context followed by ExecuteAsync

q1b

but the ExecuteAsync returns Object reference not set to an object, yet all the params for the WorkflowExecutionContext look fine.

Any pointers would be appreciated.