Check that list data is sent

Author: nateconley

mailchimp.php

@@ -634,7 +634,11 @@ function mailchimp_sf_save_general_form_settings() {
  * Sees if the user changed the list, and updates options accordingly
  **/
 function mailchimp_sf_change_list_if_necessary() {
-	if ( empty( $_POST['mc_list_id'] ) ) {
+	if ( ! isset( $_POST['mc_list_id'] ) ) {
+		return;
+	}
+
+	if ( isset( $_POST['mc_list_id'] ) && empty( $_POST['mc_list_id'] ) ) {
 		$msg = '<p class="error_msg">' . esc_html__( 'Please choose a valid list', 'mailchimp' ) . '</p>';
 		mailchimp_sf_global_msg( $msg );
 		return;