Comment 2 for bug 1260586

This is a bug of the used theme engine. It cannot render multicolumn dropdown boxes and crashes because of this.

You can fix it by installing the current version of gtk3-engines-oxygen 1.0.x
https://answers.launchpad.net/grub-customizer/+faq/2469

In future releases I'm considering changing the default operating system chooser so it will have one column only (however this is for other reasons too. In this case it's a bug of the theme engine which can also affect other applications).