No, a decree of nullity is not required of a divorced person (who is single) in order to enter the Church. If a non-Catholic had divorced and remarried, this would be a different matter.
However, even for divorced persons who are not "remarried", often priests and others associated with RCIA will *encourage *a non-Catholic to go through the nullity process before, or simultaneously with, their inquiry into the Church.
This is because there is always a chance that the marriage will be found VALID and the non-Catholic needs to understand what that means-- they can **never **marry as long as their spouse is alive.
Many non-Catholics will decide not to become Catholic when presented with this hard reality or worse, those who've already entered the Church will leave!
So, wisely, a priest might counsel a person inquiring into the faith to go through the nullity process-- if there are sufficient grounds-- so that they can know their canonical status with certainty.
But, again, it's not *required *that they **ever **petition for a decree of nullity.