Almost all IRA agreements make the default beneficiary the estate of the decedent. Similarly, 401(k) agreements usually make the estate of the decedent the default beneficiary, except when there is a surviving spouse and the 401(k) is subject to ERISA which makes the surviving spouse the default beneficiary unless the surviving spouse previously waived that right. If the estate is the beneficiary, it's subject to probate.
Creditor protection is generally provided to the retirement account participants, not to beneficiaries.
