Allow mechanisms to be linked, and allow storing links in schematics.

mods/artifact_player/radial_menu.lua

@@ -38,7 +38,7 @@ end
 
 function ns.dismiss_radial_menu(m, name)
     -- This is in case we only want to close a specific menu while leaving others intact.
-    if name and m._menu.name ~= name then return end
+    if name and (not m._menu or m._menu.name ~= name) then return end
     for _, x in ipairs(m._menu) do
         x:remove(m)
     end