EventDispatcherInterface not found

[Adrian Kiesthardt]

Hey,

I get the following error when I try the code provided by the Read Me:

Warning: 
require_once(/Users/creadisc/GIT/test/Psr/EventDispatcher/EventDispatcherInterface.php):
failed to open stream:
No such file or directory in /Users/creadisc/GIT/test/index.php on line 6

Fatal error: require_once():
Failed opening required 
'/Users/creadisc/GIT/test/Psr/EventDispatcher/EventDispatcherInterface.php' 
(include_path='.:/usr/local/Cellar/php@7.2/7.2.20/share/php@7.2/pear') in 
/Users/creadisc/GIT/test/index.php on line 6

This error occurs as soon as I instantiate a StateMachine object.
When I check the /vendor folder I cannot find any element named Psr/EventDispatcher/EventDispatcherInterface.php. There are EventDispatcherInterface files related to Symphony but they reside in a different folder structure.
Btw: I'm on MacOS

Is this a bug? How can I fix this?

Thanks in advance
Adrian