Fix #46: Generated job couldn't be saved and crash with stacktrace

[alexsedova]

Definition of Done for the pull request

• [ ] Travis CI build is passing (includes unit tests and static analysis)
• [ ] You submission reviewed by one of the maintainers
• [ ] New functionality covered by unit tests
• [ ] Help messages for UI updated
• [ ] Screenshots in README.md updated
• [ ] JobDSL help updated
• [ ] Groovy script for global configuration updated

  Recomendations

  Pull request title

• Fix #X: <issue X title> in case if you have an issue
• Update for the change introduced in #X (where X is issue or pull request) in case of no issue

  Merging pull request

Please use squash merge by default unless you have a very good reason not to do so