hello
drupal will output a warning if no menu are enabled for the content type. you need to add a conditional IF block to check if the array is empty.

warning: Invalid argument supplied for foreach() in /drupal/includes/form.inc on line 1212.

would be best if you could add a entry to the selection box saying to be automatically selected in case of no menu being enabled

menus can be disabled with the module Menu Settings per Content Type