(Copiied from JENKINS-19985)
Steps to reproduce:

1. Create a new freestyle project
2. Add the Flexible publish as a post build action.
3. Save the project without any actions for a conditional action.
4. It will throw an exception, however the save happens!
   • In Jenkins 1.425, the exception is recorded only in the system log.
   • In Jenkins 1.580.1, the exception is displayed also on the browser.
5. Execute a build and it will throw the NPE.

Another way you can get this NPE:

1. Same steps as 1, 2, 3, 4.
2. Restarting Jenkins throws this NPE, however the project will fail to load and show an error about project actions.

I set this to Critical, as this cause jobs lost for the latter problem.