can we see the log? an error text like this one is not that meaningful. but i believe that you are trying to modify a part of the map generation, and this is where i may be crashing but don't quote me on that one.
I'd say that you tried to make a new gun/item/object/..., and a configuration file is missing (or an object needed in one of your configuration files are missing). That would explain the "improper conversion to VariantMap from null" part (and i saw that you wanted a gun tutorial). But C0bra5 is still right, seeing the log would really help (Edit: also, a good tutorial for solving errors).
