A fix for the error has been PR'ed. You can manually fix it by adding the following line after the use statement at line 14 of components/com_jevents/views/icalevent/view.html.php

use Joomla\CMS\Toolbar\ToolbarHelper;

and delete the include at line 31

This is a warning we have resolved in the next release, it actually causes no issues since JToolbar is included with the view we are extending anyway (it didn't back in J1.6 when it was added) so just set your Joomla! error reporting to simple if this is a development site, or set it to none for a live site.

Many thanks
Tony

Thank you very much, I was able to solve it at the suggestion of Skurvish.