Ahhjo.
I can not reproduce your error. I copied your rule A.
What does the log say when you use the full path /home/pi/opt?
Only thing i found was the path. Which has an meaningfull error in my case.
[vent.ItemStateChangedEvent] - Remote_CodeSend_Out changed from sending code[11607815] to Cannot run program "/SourceCode/433Utils/RPi_utils/codesend": error=2, No such file or directory
A null error mostly says one used element is not initialized. Try to change all files and save them again. See if the log shows an error.
Maybe you saved the rule before you had the items? Restart openhab, better the whole RPI.
Make sure the items are instanciated by looking in the openhab console or VSCode.
Do you use VSCode and the Openhab plugin?
And my setup.