Button to restrict access to chosen group is missing classes to be displayed correctly in Boost-Theme

When you open the settings page of an activity-module using groups/groupings (e.g. mod_assign) and having restricted access enabled a button is displayed to restrict access to current chosen group. This gets handled in moodleform_mod::standard_coursemodule_elements(). The button-HTML is created without the classes "btn btn-primary" which makes it appear completely unformatted.