PHP Warning: Declaration of aec_contributor_list::form() should be compatible with WP_Widget::form($instance)

WP_Widget::form() expects to receive an array as a parameter, but the plugin’s form() function doesn’t have it.

You can probably fix it like this:

/** @see WP_Widget::form */
function form ( $instance ) {              
    _e('No options available.', AEC_NAME);
}

Be aware, though, that if the plugin’s not supported by its developers anymore, there won’t be any security updates.