Fatal error when submitting form

The plugin throws a fatal error:

PHP Fatal error: Uncaught TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given in wp-content/plugins/contactsheets-lite/includes/class-wpsslc-service.php:886

wp-content/plugins/contactsheets-lite/includes/class-wpsslc-service.php(336): WPSSLC_Service->check_mapping_fields()