JSON error for certain units when locking/unlocking lessons

[breity]

Description

Certain projects fail to lock or unlock lessons. Haven't been able to determine what causes this, as onnly specific units have the problem. Has something to do with unit json, as AngularJS throws this error:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Subscriber'
    |     property '_subscriptions' -> object with constructor 'Array'
    |     index 0 -> object with constructor 'SubjectSubscription'
    --- property '_parentOrParents' closes the circle
    at Object.stringify (<anonymous>)
    at Object.toJson (angular.js:1458)
    at TeacherProjectService.saveProject (projectService.ts:1144)
    at NavItemController.toggleLockNode (navItem.ts:293)
    ...

Steps to Reproduce

1. Log in as a teacher and launch Teacher Tools for a run.
2. Click the lock icon on one of the lessons.
3. Locking will fail for units that have this problem and you'll see the above error in the console.