The problem is in the URL which you are sending out in your email
http://localhost:3000/users/accept_invitation.tLPOjM3hdA13rEv5FNhsOQ
In the above link, there are no params generated and thus when the first line of the accept_invitation action runs, then @user is set to nil because there are no params[:email_token].
According to me, you should set your link something like
registration_confirmation.html.haml
Confirm your email address please!
= accept_invitation_users_url(:email_token => @user.email_token)
in your mailer view