Call to a member function end() on null in symplify

[sakarikl]

Bug Report

Subject	Details
Rector version	Rector 1.2.5

PHP Fatal error: Uncaught Error: Call to a member function end() on null in rector/vendor/rector/rector/vendor/symplify/easy-parallel/src/ValueObject/ParallelProcess.php:120

I'm not sure if this problem is with rector or with symplify/easy-parallel

Minimal PHP Code Causing Issue

I do not have minimal set to reproduce it. I have a large symfony 1.4 application which I am running to date with step by step guide.
Some runs produce that bug but not repeatably.

Expected Behaviour

[sakarikl]

it seems that it was caused by tecnickcom/tcpdf library. While trying to run rector on it caused those errors. That was included by accident in my configuration.