Trying to access array offset on value of type null in forum_get_topics()

Problem/Motivation

I couldn't find an existing issue for this exception.

exception: [Notice] Line 926 of modules/forum/forum.module: Trying to access array offset on value of type null

Here's the relevant code (including line numbers initially):

 914 function forum_get_topics($tid, $sortby, $forum_per_page) {                     
 915   global $user, $forum_topic_list_header;                                       
 916                                                                                 
 917   $forum_topic_list_header = array(                                             
 918     NULL,                                                                       
 919     array('data' => t('Topic'), 'field' => 'f.title'),                          
 920     array('data' => t('Replies'), 'field' => 'f.comment_count'),                
 921     array('data' => t('Last reply'), 'field' => 'f.last_comment_timestamp'),    
 922   );                                                                            
 923                                                                                 
 924   $order = _forum_get_topic_order($sortby);                                     
 925   for ($i = 0; $i < count($forum_topic_list_header); $i++) {                    
 926     if ($forum_topic_list_header[$i]['field'] == $order['field']) {             
 927       $forum_topic_list_header[$i]['sort'] = $order['sort'];                    
 928     }                                                                           
 929   } 

Proposed resolution

Fix the bug; avoid the PHP exception / notice in PHP 7.4.

Remaining tasks

Patch, review, commit etc..

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

See parent issue.