Are you comfortable enough to edit the GP Premium plugin?
If so, go to
wp-content/plugins/gp-premium/menu-plus/functions/generate-menu-plus.php and find this line:
'before_widget' => '<aside id="%1$s" class="slideout-widget">',
Then replace it with this:
'before_widget' => '<aside id="%1$s" class="slideout-widget %2$s">',
That should fix the issue until we release the update. We don’t have a 1.7.8 planned, so you shouldn’t have to worry about losing the change on update. However, anything can happen, so keep an eye on it 🙂