Crash while doing background subtraction

[cmeyer]

Reported on Twitter.

Needs follow-up.

• How has Swift been installed?
• What are the specs of the machine (CPU/memory/storage-type)?
• What steps reproduce the issue? If it is intermittent, what is a typical use case when the problem occurs?
• If this is occurring on an SI, how is the SI being imported? (is it backed by ndata or h5py)?