Parameter must be an array

Hi,
I am testing your plugin Pie Register if if fits me needs. But when calling Pie Register / Notifications / User Notification I get this error on top of Your account Is Ready:
( ! ) Warning: sizeof(): Parameter must be an array or an object that implements Countable in C:\xampp\htdocs\vitaring\wp-content\plugins\pie-register\menus\notification\PieRegUserNotification.php on line 7
Call Stack
# Time Memory Function Location
1 0.0011 449080 {main}( ) …\admin.php:0
2 4.2203 147517744 do_action( ) …\admin.php:259
3 4.2203 147518120 WP_Hook->do_action( ) …\plugin.php:484
4 4.2203 147518120 WP_Hook->apply_filters( ) …\class-wp-hook.php:311
5 4.2203 147519248 PieRegister->PieRegNotifications( ) …\class-wp-hook.php:287
6 4.2203 147519360 PieRegister->include_pr_menu_pages( ) …\pie-register.php:4295
7 4.2203 147519360 PieRegister->require_once_file( ) …\base.php:2872
8 4.2206 147529904 require_once( ‘C:\xampp\htdocs\vitaring\wp-content\plugins\pie-register\menus\PieRegNotifications.php’ ) …\base.php:2487
9 4.2207 147530032 PieRegister->require_once_file( ) …\PieRegNotifications.php:30
10 4.2214 147571384 require_once( ‘C:\xampp\htdocs\vitaring\wp-content\plugins\pie-register\menus\notification\PieRegUserNotification.php’ ) …\base.php:2487
What’s the problem here?
WP = 5.6.2, Pie Register = 3.6.16
Thx
Oliver

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