A crack at the end of the clear storage cap (where the sponge is located) does not affect the functionality of the Vernier Optical DO sensor.
A crack can commonly occur due to over-tightening the end of the storage cap. When placing the storage cap on make sure that it is secure, but not too tight.
See image below for an example.
If the cap was damaged by over-tightening, replacements are sold as Optical DO Storage Bottle Cap (ODO-SC).