Having an error in the script prevents from adding a new script as a state behaviour

Repro:
1. Select a state in the Animator Controller
2. Add a new behaviour script to it
3. Duplicate the script in the Project browser - an error will pop up
4. Select another state and try to add a new behaviour script to it - the new script will be added to the Project, but it will not show in the state's Inspector
5. Delete the duplicated script
6. go back to the second state - the script will show now