Sorry, there is no deductible loss here, for two reasons.
1. Theft is not deductible for 2018-2025 as a result of the 2017 tax reform, unless the theft occurred as part of a federally declared disaster of some kind.
2. Even if theft losses were deductible, your loss is not deductible because you have no basis in the 401k. All the contributions and growth was tax-free when it occurred, and whatever you withdraw is taxable. Your "tax reduction" is that fact that you won't pay taxes now because you have less to withdraw. You can't deduct something from your taxable income if it was never included in your taxable income in the first place.
