Steps to reproduce:

1) Ensure that SMTP server is set and public signup is on
2) Create a review
3) In the "invite reviewers" press invite with a blank text field
4) In the "invite reviewers" press invite with "test" in the text field.

Both of these appear to be added with no error message, despite the fact that both are not valid email addresses.

Also, add a check for an empty/undefined email address and also make the error reporting clear as to what the error is either on screen or in the logs.