i face this issue on weekend so i try my best before submit his ticket i already try in new project and its work fine but on project this bug for some reason shown, on my project the 1.5V was work fine but after upgrade from 1.5 to 1.7.7 i face it (That post looks like a different error. I'm confident that the error that you are getting has been fixed. Can you try creating a fresh project, download and import Behavior Designer from the Asset Store, and then try again?
after more investigation between the versions i detect 2 serialization different Items :I will send you a PM with the version that I have. I don't have too many older versions but do have a backup of 1.6.1 which I hope you'll first be able to update to with your tree.