Seems to be related to r84007 which attempted to fix bug 27887.

mhershberger wrote:

Unassigning default assignments. http://article.gmane.org/gmane.science.linguistics.wikipedia.technical/54734

Still happening.

• Bug 37333 has been marked as a duplicate of this bug. ***

Line number has changed (or another issue?) though: PHP Notice: Undefined variable: _SESSION in /www/translatewiki.net/w/extensions/LiquidThreads/classes/View.php on line 415

Still happening.

Today quite a spree of these.

This was attempted to be fixed in 2011:

https://github.com/wikimedia/mediawiki-extensions-LiquidThreads/commit/f2caf1cdf20c3604fe1e635b9d8e869575f751fa

But that only made it worse by now iterating over array(null) instead of not iterating and emitting a notice for loop over non-iterable.

Change 136012 had a related patch set uploaded by Krinkle:
View.php: Fix notice for undefined _SESSION

https://gerrit.wikimedia.org/r/136012

Change 136012 merged by jenkins-bot:
View.php: Fix notice for undefined _SESSION

https://gerrit.wikimedia.org/r/136012

Patch mentioned in this report was merged - assuming this bug is FIXED.
If that is not the case: Please reopen and elaborate what is left to do here to get this report fixed.