fix the import issue when 'box_ignore_box_vagrantfile' is set, the box object should never be null.
fix the import issue when 'box_ignore_box_vagrantfile' is set, the box object should never be null.

bryson/vagrant,marxarelli/vagrant,sni/vagrant,gitebra/vagrant,mitchellh/vagrant,chrisroberts/vagrant,bryson/vagrant,mitchellh/vagrant,marxarelli/vagrant,gitebra/vagrant,gitebra/vagrant,marxarelli/vagrant,sni/vagrant,marxarelli/vagrant,sni/vagrant,gitebra/vagrant,chrisroberts/vagrant,mitchellh/vagrant,chrisroberts/vagrant,bryson/vagrant,mitchellh/vagrant,sni/vagrant,chrisroberts/vagrant,bryson/vagrant