error in view.html.php after Joomla upgrade to 3.7.1

After upgrading Joomla from 3.7.0 to 3.7.1 - jem 2.2.1 calendar display shows the error message:

Notice: Trying to get property of non-object in /components/com_jem/views/calendar/view.html.php on line 88

The statement in error is:

$pagetitle = $params->def('page_title', $menuitem->title);

var_dump($menuitem->title) gives NULL !

What can I do?

I tried to reproduce your issue: but I didn't succeed. I updated too from joomla 3.7.0 to 3.7.1 with JEM 2.2.1.
It's a clean install with no others models,plugins or templates.
All is working fine, no such error notification. Did you change something?

Thank you for your reply.
No - I did not change anything but Joomla.
Check this out:
production Server with Joomla 3.7.0
www.diebrueckenbaeckerei.de/index.php?option=com_jem&view=calendar&Itemid=495
Test Server wit Joomla 3.7.1
bluert.ddns.net/bruecke/index.php?option=com_jem&view=calendar&Itemid=495

Might it be a Joomla issue?

you did not answer.
Do you have installed other things than joomla 3.7.1 and his official modules,templates and plugins beneath the JEM extension, modules and plugins? If so then perhaps these things were not prepared for Joomla 3.7.1.

A propos: If I choose these urls I get no calendar but "theendofinternet". I'ne never seen that. Is this yours? and where is the calendar?

I did not install/change anything but Joomla today.
sorry for the end of the internet - it is to protect the site against hackers. Access is only permitted to clients located in Germany.
But I am using the FAVOURITE template by FAVTHEMES.
I am going to try it with an original joomla template.
Thank you for now.

Must be something other: I installed favourite from favthemes: calendar is working fine!