Notice: Undefined variable: res in wp-fastest-cache/inc/cdn.php on line 169

Getting the title PHP notice when saving an Advanced Custom Field. ACF Pro, saving a field assigned to an options page.

Actually get two notices:

Notice: Undefined variable: res in .../plugins/wp-fastest-cache/inc/cdn.php on line 169
Notice: Trying to access array offset on value of type null in .../plugins/wp-fastest-cache/inc/cdn.php on line 169

And then a headers could not be sent error.