Undefined index: HTTP_HOST error

When debug.log is turned on we are getting this error/warning thousands of times a day.

[15-Aug-2019 17:34:02 UTC] PHP Notice: Undefined index: HTTP_HOST in ……/wp-content/plugins/ultimate-member/includes/core/class-permalinks.php on line 140

Host: Siteground – Cloud server
WordPress: 5.2.2
Ultimate Member: Version 2.0.54
PHP Version: 7.1.30

Several plugins like Woocommerce are in use but I doubt the error is related.

Below is the code where the error is occurring (line 140). It appears to be somehow related to WP-CLI.

Any thoughts? It is generating so many errors / warnings in the debug.log file that it makes the file very large in no time.

137  if ( defined( 'WP_CLI' ) && WP_CLI ) {
138      $host = isset( $_SERVER['HTTP_HOST'] ) ? $_SERVER['HTTP_HOST'] : 'localhost';
139  } else {
140      $host = $_SERVER['HTTP_HOST'];
141  }