Unfortunately it seems like the only fix right now is to not destroy the Fabric material resources. This will cause a leak, but only when material pools are destroyed which doesn't happen during normal usage, only when you change certain /Cesium properties or reload the extension.
Fixes #444
An old bug has shown up again. If material pools and their associated Fabric prims are destroyed, and new prims are created later, it causes a crash. https://github.com/CesiumGS/cesium-omniverse/issues/444#issuecomment-1936520180 goes into more detail with reproduction steps.
Unfortunately it seems like the only fix right now is to not destroy the Fabric material resources. This will cause a leak, but only when material pools are destroyed which doesn't happen during normal usage, only when you change certain
/Cesiumproperties or reload the extension.