Use get_mapgen_setting.

init.lua

@@ -67,15 +67,27 @@ end
 
 -- whether to use biomes and heightmap
 fun_caves.use_bi_hi = false
+local mgname
+if minetest.get_mapgen_setting then
+	mgname = minetest.get_mapgen_setting('mg_name')
+else
 	local mg_params = minetest.get_mapgen_params()
-local mgname = minetest.get_mapgen_setting('mg_name')
+	if mg_params then
+		mgname = mg_params.mgname
+	end
+end
+
 if mgname ~= "v6" and mgname ~= "v5" then
 	fun_caves.use_bi_hi = true
 end
 
 
 minetest.register_on_mapgen_init(function(mgparams)
+	if minetest.set_mapgen_setting then
 		minetest.set_mapgen_setting('mg_flags', "nocaves, nodungeons", true)
+	else
+		minetest.set_mapgen_params({flags="nocaves,nodungeons"})
+	end
 end)