I tried running it without any other widgets in a clean 7.0.2 project and It worked fine for me.
In the console was a message (debug: true)
mendix.lib.MxObject(MyFirstModule.Task:4222124650659843).get: No attribute
But no visible problems.
Edit 1: you are right, you must the first one to find this. The widget has an error
the line in calendar-scheduler.js
return declare("calendar.widget.calendar", [calendarWidget], {
must be
return declare("calendar.widget.calendar-scheduler", [calendarWidget], {
I made a pull request, but in the meantime you could possibly patch the widget yourselves.