Hello there @jppaceman!
From what I could gather around the community, this type of error (138), usually pops up in cases of bad memory access, or plugin conflicts (the most likely case).
To tackle this one, it boils down to ensure you have the latest version of everything. Confirm you are using the latest version of Cesium, .NET 6 SDK, and Xcode. for UE. As well, please make sure all these elements are compiled into your project (confirm it’s listed into your .uplugin file).
After that, if the issue persists, to isolate the cause, test packing without Cesium included.