Hello,
the property cm:subjectline may simply not be set which will result in errors when you attempt to create a folder without providing a name (or even an invalid name, as the value of cm:subjectline can be incompatible with the required file / folder name). I'd suggest adding the necessary checks to your script, i.e. verify that cm:subjectline actually has a value and then attempt to lookup / create a folder based on it - additionaly, you must make sure to sanitize cm:subjectline to comply with cm:name constraints about allowed format.
Regards
Axel