Cannot read property 'children' of undefined when spawned machine is at the final state

[github0013]

Description
When you have multiple levels of machines in context by spawning machines, if a grand child machine's state is final, you get an error saying Cannot read property 'children' of undefined.

Expected Result
This error should not happen.

Actual Result
Cannot read property 'children' of undefined error occurs.

Reproduction

parent -> child -> grand child (final)
https://codesandbox.io/s/reverent-austin-uztit

parent -> child (final) this one won't throw the error.
https://codesandbox.io/s/empty-paper-s2l4u