null pointer dereference

Somewhere between commits:

`40fb2468886b710fd1acaf` and `0a13079b670ce2924e4a81a141`

Core is now failing, sporadically but reproducibly, with a null pointer de-reference error. 

Two crash logs below.

I've changed some code in this area, recently, as well: the lambdas are run serially, but not necessarily on a single thread.  None the less, I have never seen this error at commit `40fb2468886b710fd1acaf` and it always occurs at `0a13079b670ce2924e4a81a141`