In rare scenarios, it is possible for multiple ESXi Hosts to crash while performing an on-disk format version (DFC) upgrade on a stretched cluster. This PSOD is most likely to occur if the witness ESXi Host was replaced during the upgrade process and DFC was run shortly after. This could trigger a race between DFC and witness replacement workflow.
This can result in executing an unexpected code path and causing a null pointer exception leading to the PSOD.