There has been lots of Robinias dying from a root disease and my local garden centre has stopped selling them. Gleditsia triacanthos (Honey Locust) would be my choice as a substitute. If you are going to replant in the same or general area then perhaps use Rootgrow in the planting hole just to be on the safe side. Might not be a bad idea to remove some of the surrounding soil from the Robinia.