Symfony 4: Var-dumper not working properly

Question

I tried to use the wonderful dump function from the var-dumper bundle in symfony 4 and for some reason I get the following error:

Failed to start the session because

Answer 1

A guy named Simon Waldburger mentioned in the lesson's comments that installing "debug" fixed the issue (it fixed it for me). The instruction was in the next lesson.

composer require debug --dev

iiirxs is also correct, to output dump in twig file, do

{{ dump(variable) }}