Certain roots in fly crash on proofreading changes

[manuel-castro]

Several users have reported that they are unable to split certain segments in fly. The error message they receive from the ChunkedGraph is "Split failed: int() argument must be a string, a bytes-like object or a number, not 'NoneType'".
Example segments: 720575940611721169, 720575940621143607. Links posted on a thread in the flywire forum.

Previously we ran into this error in the Minnie CG and @akhileshh fixed a bug that was related to skip connections. See: #236. So it's plausible that it's the same or related bug in the master branch.