PHP Error when viewing post

Uncaught Error: Non-static method mijnpress_plugin_framework::get_plugin_url() cannot be called statically in /www/site/public/wp-content/plugins/see-attachments/see-attachments.php:66
Stack trace:
#0 /www/site/public/wp-admin/includes/template.php(1409): plugin_see_attachments_inner_custom_box(Object(WP_Post), Array)
#1 /www/site/public/wp-admin/edit-form-advanced.php(711): do_meta_boxes(Object(WP_Screen), 'advanced', Object(WP_Post))
#2 /www/site/public/wp-admin/post.php(206): require('/www/site...')
#3 {main}
thrown