Thanks for the workaround.
For anyone who got confused with the template name as me (I thought it should be as my WP template name). Just copy the file to all of the folders under the theme folder.
Take this file: wp-content/plugins/all in one calendar/public/admin/agenda-widget-form.php
I’ve copied it to all themes folders:
wp-content/plugins/all in one calendar/public/themes-ai1ec/vortex/
wp-content/plugins/all in one calendar/public/themes-ai1ec/gamma/
wp-content/plugins/all in one calendar/public/themes-ai1ec/plana/
wp-content/plugins/all in one calendar/public/themes-ai1ec/umbra/
Then deactivated and reactivated, and the error message has gone. Good luck with that.