php error in Undefined variable: style_output on line 133

I made a clean install, went to the member page and got:
Notice: Undefined variable: style_output in /var/www/sites/zzphuizen/www.zzp-huizen.nl/htdocs/wp-content/plugins/simple-staff-list/_inc/user-view-show-staff-list.php on line 133

I’m running standard wordpress (newest) and twenty twelve theme.

Example:
http://www.zzp-huizen.nl/?page_id=4

No idea how to define the variable, from the sourcecode:

132 wp_reset_query();
133 $output = $style_output.$output;
134 return do_shortcode($output);

If I comment it out, it doesn’t give an error (so $output can be undefined?)

Did I miss something basic?

https://ww.wp.xz.cn/plugins/simple-staff-list/