Adds a missing not-null check that is made in another case.

code/game/atoms_movable.dm

@@ -164,7 +164,7 @@
 			a = get_area(src.loc)
 	else
 		var/error = dist_y/2 - dist_x
-		while(src && target &&((((src.y < target.y && dy == NORTH) || (src.y > target.y && dy == SOUTH)) && dist_travelled < range) || (a.has_gravity == 0)  || istype(src.loc, /turf/space)) && src.throwing && istype(src.loc, /turf))
+		while(src && target &&((((src.y < target.y && dy == NORTH) || (src.y > target.y && dy == SOUTH)) && dist_travelled < range) || (a && a.has_gravity == 0)  || istype(src.loc, /turf/space)) && src.throwing && istype(src.loc, /turf))
 			// only stop when we've gone the whole distance (or max throw range) and are on a non-space tile, or hit something, or hit the end of the map, or someone picks it up
 			if(error < 0)
 				var/atom/step = get_step(src, dx)