[migration] fix bug in `CWAttributeAddOp.revertprecommit_event`
The `rdefdef` attribute is set during the operation execution. In some the
operation crash before this assignment. In such case `revertprecommit` raise an
attribute error crashing the whole process and shadowing the original error.
This changeset detect and Avoid this situation.
Schema of the data------------------First take a look at the data schema_ then try to remember that you are browsingthrough a heap of data by applying stylesheets to the results of queries. Thissite is not a content management system with items placed in folders. It is aninterface to a database which applies a view to retreived data..._schema: schema