If I am not wrong udev is responsible for creating /dev/null as a virtual node. I think there is something happening like race condition in system. thread is making an attempt to access the file before the thread has been created. Mostly it works perfectly but sometimes sync issue occurs and nodes were not available when it is requested.
To resolve the issue I recommend below command and let me know whether it is working or not.
sudo rm -f /dev/null
sudo mknod /dev/null c 1 3
sudo chmod a+w /dev/null