Creating default object from empty value - Sprout Email
This error can be triggered when you try to use a variable name in your Sprout Email template or fields that is not present in the submitted Form data.

One example where this can occur is if you try to reuse the example Welcome Notification email provided in Sprout Email and don't update the {email} or {username} values in the HTML and Text body fields.

I'll add it to our list to revisit the error thrown here, as it's not clear at all what is happening from the error provided in the message you received above.