If you were calling the_widget("ai1ec_agenda_widget"…)

…in a theme file, you’ll need to swap that out with:

the_widget('Ai1ec_View_Admin_Widget',array(options go here))

With the installation of 2.1.2 (and possible a version or two previous).

https://www.ads-software.com/plugins/all-in-one-event-calendar/