UBB.threads PHP Forum Software Community Forums UBB.threads 7 Installation and Upgrade Support Undefined variable on External Post Island [7.6.0]

I think I have fixed it myself.

Control Panel -> Language -> "Search / Edit Language Strings" tab -> select generic.php and search.
Do nothing on this page, just click "Update Language File" button near the bottom of this page.

Error disappear. Thanks for helping.