NullPointerException when changing build strategy for a plan

Steps:
1) go to plan configuration -> source repository
2) change build strategy to 'Polling the repository for changes' (in my case, from 'Manual & Dependent builds only')
3) Change polling frequency (in my case, from 180 to 300s)
4) hit 'Save'

Result: server error (NullPointerException).
Attached screenshots and stack trace.