Is my divorce decree still effective if the ex-wife is deceased?

The Decree should have a line in it that states that you retain the right to custody in the event of your ex-wife's death - or something along those lines. Even if it doesn't and unless custody has been awarded to a third party by a court, you automatically become the primary custodian of your child. If there is a third party that is claiming custody, that person would have to file something with the court to exercise custody. Absent any court ruling you now have full custody.