Code:

mob
var
heal
spell
magic
verb
Fire()
var/colour = "red"
set category="Magic"
if(spell) return //if the usr is already using a spell return
spell=1 //the usr is using a spell
var
direct=dir //makes direct equivulant to usr.dir
obj/spells/Fire/O=new(usr.loc) //creates fire where the usr stands
damage = ((usr.str / 3) + usr.str) //the damage that the fire will cause
location //varible defined later...
spawn(30) //do the following in 3 seconds
spell=0 //the usr is not using a spell
if(O) //if O still exists
del(O) //delete the fireball
while(O) //while O exists
location=O.loc //location is the fireballs location
sleep(2) //the speed of the fireball
step(O,direct) //fireball moves in "direct" direction
if(location==O.loc) //if location is still equal to the fireballs loc after the step proc, then delete it.
del(O)
for(var/mob/M in view(0,O))
if(M==usr) //if M is usr
continue //restart the for proc
if(M.density) //if the mob has density
M.hp-=damage-M.def //add damage and spawn the death proc
spawn()
s_damage(M,damage,colour)
spawn()
M.DeathCheck()
del(O) //delete the fireball and return
return
for(var/obj/o in view(0,O))
if(o.density) //if o has a density delete the fireball and return
del(O)
return
for(var/turf/t in view(0,O))
if(t.density) //if t has a density delete the fireball and return
del(O)
return
continue
Problem description:

MagicMan (/mob/character): Fire()
runtime error: Cannot read null.loc
proc name: Blizzard (/mob/magic/verb/Blizzard)
usr: MagicMan (/mob/character)
src: MagicMan (/mob/character)
call stack: