Recoverable fatal error: Object of class SiteActivity could not be converted to string in filter_xss() (line 1442 of /common.inc

I had the site activity blocks placed on a panel page, and when returning to edit the panel content, I received this fatal error:

Recoverable fatal error: Object of class SiteActivity could not be converted to string in filter_xss() (line 1442 of /.../includes/common.inc

I found that when disabling the Heartbeat activity defaults submodule the fatal error goes away and allows me to view and edit the panel's content.

Here are some other errors being reported but they persisted whether the fatal error with the defaults submodule was enabled or not but did not keep me from editing the panel's content:

Warning: strlen() expects parameter 1 to be string, array given in drupal_validate_utf8() (line 1587 of /.../includes/bootstrap.inc).
Warning: strlen() expects parameter 1 to be string, array given in drupal_validate_utf8() (line 1587 of /.../includes/bootstrap.inc).
Warning: strlen() expects parameter 1 to be string, array given in drupal_validate_utf8() (line 1587 of /.../includes/bootstrap.inc).
Notice: Array to string conversion in filter_xss() (line 1442 of /.../includes/common.inc).
Warning: strlen() expects parameter 1 to be string, array given in drupal_validate_utf8() (line 1587 of /.../includes/bootstrap.inc).

The fatal error in question did not rear it's ugly head months ago, so what can be done about it now after multiple Drupal and other module updates?