Well, I hoped you could help me with this. If I import the source code package on a brand new project I get the error you said, which disappears after reimporting the editor (it's just the Editor dll is being imported before the rest of the package), but when I try to create an External Behavior Tree I get a null reference, so I can't even test it.
I'd rather like to check things on the current project, which worked well before the update. I guess you could know where the problem is if an external behavior tree doesn't throw any debug information on runtime (nodes should go green, current shared variables values should be shown...).