IsSpellKnown(108853)) is returning false when on a Fire Mage. 108853 is the spell id for Fire Blast. This is causing IsSpellInRange to return nil.
Usually, in my experience Lua errors are due to an addon.
Do us a favor, try a full and complete UI reset. Just rename the folders so if it does not help you can rename them back
If fhe error goes away you have an addon not working properly. If you look close enough in the Lua error I do believe it tells you which one. Other than that it is process of elimination.
Hope this helps and works for you bud.
A proper reset UI would be the best first step. Luas usually are addon related.