Your Site is Experiencing a Technical Issue

Since a while I have an error with WordPress and Popup Maker. The error is shown in this automatic message fom WordPress. However, there is no error on the site I can realize and the login to WP is only used by myself. That’s why I have ignored the message and was hoping on the next update… but the WP update as well as the Popup Maker updates didn’t changed anything.

This is the content of the WordPress mail I allways recieve:

Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.
In this case, WordPress caught an error with one of your plugins, Popup Maker.

First, visit your website (https://domain.com/) and check for any visible issues. Next, visit the page where the error was caught (https://domain.com/wp-login.php) and check for any visible issues.

When seeking help with this issue, you may be asked for some of the following information:
WordPress version 5.7
Current theme: (version )
Current plugin: Popup Maker (version 1.16.1) PHP version 8.0.3

An error of type E_ERROR was caused in line 173 of the file /home/www/wordpress/wp-content/plugins/popup-maker/classes/Site/Popups.php.
Error message: Uncaught Error: Attempt to modify property “posts” on null in /home/www/wordpress/wp-content/plugins/popup-maker/classes/Site/Popups.php:173