Line 97 of extension/ezflow/classes/ezflowpool.php

if ( $validNodeObject->canRead() )

It may cause fatal errors in the frontend if the object is null.
In that way, it makes sense that validNodeObject is checked as an instance of ezcontentobjecttreenode before the test, what was implemented in:
https://jira.ez.no/browse/EZP-21675.

But there's still de need of a backport to eZ Publish 5.2