here https://github.com/angular/zone.js/blob/9c65d25f583548d8366d94a633a8fceaecc6d1ca/lib/rxjs/rxjs.ts#L120
_unsbscribe is applied before null check
for retryWhen Operator it is set to be null for brief time see here

https://github.com/ReactiveX/rxjs/blob/d229235f4812dd5140209300e7cddf55553d5f53/src/internal/operators/retryWhen.ts#L108