Warning: Trying to access array offset on value of type null in calendar_preprocess_date_views_pager()

Problem/Motivation

Warning: Trying to access array offset on value of type null in calendar_preprocess_date_views_pager() (line 88 of .../sites/all/modules/calendar/calendar.module).
and
Warning: Undefined array key "menu" in calendar_preprocess_date_views_pager() (line 88 of .../sites/all/modules/calendar/calendar.module).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes