Two things are happening. First, there is no error (in your example) for that field.
Second, there is a bug (in some sense) in that the related items aren't set to '', leading to the error. I'll look into fixing it before the release of 1.7
So, a short term fix is to use isset() before echoing them.
Update: That was easy. If you want to manually fix 1.6.2, just go to the clear() method, and add this line to each of the $has_one and $has_many foreach loops: