If you are selecting opponents for your game and you select one to read the history of them and then go into the civlopedia it will crash somtimes. I experienced this when reading about Egypt, then clicking for more information on their war chariots, then going to the main units list and clicking archers. When I click on archer the game crashed.

Do you have a save to reproduce, or some steps on how it happened? Was this a scenario, so maybe the entry got deleted?

Check your Civilopedia.txt file in your ../Conquests/Text/ folder to see if that entry is there.