Describe the bug

I installed cacti 1.2.20 and faced the error. Can you help me explain and fix this bug:

2022-04-29 11:32:15 - ERROR PHP DEPRECATED: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in file: /var/www/html/cacti/lib/variables.php on line: 219
2022-04-29 11:31:21 - CMDPHP PHP ERROR Backtrace: (/host.php[134]:host_edit(), /host.php[743]:draw_edit_form(), /lib/html_form.php[114]:draw_edit_control(), /lib/html_form.php[303]:form_callback(), /lib/html_form.php[831]:html_create_list(), /lib/html.php[1041]:null_out_substitutions(), /lib/variables.php[219]:preg_replace(), CactiErrorHandler())
2022-04-29 11:31:21 - ERROR PHP DEPRECATED: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in file: /var/www/html/cacti/lib/variables.php on line: 219_