Weird error I've been getting

[Jorm]

A few times recently when I've opened a topic on the forum in a new tab I've gotten this message across the top of the page.

Warning: Illegal string offset 'member_group_id' in /home/sneezefe/public_html/forums/admin/applications/forums/sources/classes/topics.php on line 2062

Warning: Cannot modify header information - headers already sent by (output started at /home/sneezefe/public_html/forums/admin/applications/forums/sources/classes/topics.php:2062) in/home/sneezefe/public_html/forums/admin/sources/classes/output/formats/html/htmlOutput.php on line 114

Warning: Cannot modify header information - headers already sent by (output started at /home/sneezefe/public_html/forums/admin/applications/forums/sources/classes/topics.php:2062) in/home/sneezefe/public_html/forums/admin/sources/classes/output/formats/html/htmlOutput.php on line 127

Everything else loads fine below that, including the header, so I'm not sure what it's all about. It only happened occasionally and everything works fine, so I'm not to worried about it, but the use of the word "warning" is a little concerning, so I figured I'd ask.