[Term Reference Tree] Widget form elements displaying as toggles instead of checkboxes

Problem/Motivation

When using the Taxonomy Term Reference Tree Widget → module, the Gin displays the form elements as toggles instead of checkboxes.

Steps to reproduce

1. Install and enable the Taxonomy Term Reference Tree Widget
2. Create a term reference field and set the widget to Term Reference Tree.

Note that Seven and Claro both display the form elements as checkboxes.