Fix to avoid error when clear cascading field

[grcreutzberg]

Hello I work with this extension and it helps me in several scenarios. I took the liberty of using this project as a base to create other extensions to deal with the mandatory nature of some fields in my Work Items. However, I found an error when clearing a field that has a cascading rule, which could cause conflict with other implementations.

Please check this pull request, I'm grateful to be able to contribute to the project.