removing an inherited attribute class throws 'uncaught exception'

[rvdb]

This is new in Roma-0.3.0, and I'm guessing it might be related to #101.

When an ODD is create from scratch in Roma, removing (some) inherited attributed classes throws an uncaught exception: [object Object] error.

Following steps illustrate the issue:

1. start with TEI Minimal schema
2. select <persName>
3. delete att.datable.custom from <persName>

This throws the error mentioned above.

Note: the same error is thrown when att.datable.iso is removed. Yet, removing att.datable.w3c does work.