callback, function ” not found

This doesn’t seem to cause problems but I was wondering if anybody can shed some light on what those below PHP warnings may cause?
The first one appears when I click on the ‘Contacts’ link in the WP Backend and all four show up when I save a form.

Any info is much appreciated.

Thanks,
olaf

[03-Oct-2019 10:11:06 UTC] PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ” not found or invalid function name in

/var/www/html/wp-includes/class-wp-hook.php on line 288
[03-Oct-2019 10:11:06 UTC] PHP Notice: Undefined index: wpcf7pdf-use-plugins-css in /var/www/html/wp-content/plugins/wp_cf7_pdf_generator_1.3.3.zip/admin.php on line 451
[03-Oct-2019 10:11:06 UTC] PHP Notice: Undefined index: wpcf7pdf-use-of-html in /var/www/html/wp-content/plugins/wp_cf7_pdf_generator_1.3.3.zip/admin.php on line 454
[03-Oct-2019 10:11:06 UTC] PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ” not found or invalid function name in /var/www/html/wp-includes/class-wp-hook.php on line 288
[03-Oct-2019 10:11:06 UTC] PHP Notice: Only variables should be passed by reference in /var/www/html/wp-content/plugins/wp_cf7_pdf_generator_1.3.3.zip/admin.php on line 105