Cracks don't mean the death of the instrument any more since the beneficial invention of the second adhesive (Cyanoakrylat).
If You should see a crack in your instrument buy a new tube second adhesive (no gel but a tube that content is as fluid as possible. ( that means: viscosity as large as possible). Try to open the crack carefully a little and let second adhesive amply flow into the crack. After two days the adhesive is hardened and you can smooth the surface if You want. With a little luck at dark woods the crack will not longer to be seen. At box-tree You will still see it. If it didn't work, try it again.
Also small holes, as they are frequently to be found in the precious box-wood, can be filled with second adhesive.