backported nil check from tenplus1 fork

* take notice from https://forum.minetest.net/viewtopic.php?p=389433#p389433
* also check previous error

arrows/dragonfire.lua

@@ -43,7 +43,7 @@ minetest.register_entity(fname, {
 							end
 						end
 					end
-		local apos = self.object:get_pos()
+		local apos = self.object:get_pos(); if not apos then return end
 		local part = minetest.add_particlespawner(
 			6, --amount
 			0.3, --time