PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback,

Hi,

We are receiving this error when using Pos.

[error] 105685#105685: *98838 FastCGI sent in stderr: “PHP message: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /public/wp-includes/class-wp-hook.php on line 286” while reading response header from upstream, client: request: “POST /wp-json/wc/v3/orders HTTP/1.0”, upstream: “fastcgi://unix:/var/run/php7.3.sock:”, host: “www.”, referrer: “/pos/”

We have disabled all other plugins except woocommerce and pos.

We are using php 7.2 and have tested with php 7.3 and the same problem appears.

We are using wordpress 5.2.1

So we would be very convinced that the error comes from pos. Everything works fine. But the error is present. So not anything fatal.

Thanks