In case you are still struggling:
I believe the post you mentioned describes the probable cause as follows:
You are using a blueprint Decorator which is using a variable of type Blackboard Key Selector and is Editable in Instances. Then in the instance of the Decorator in the Behavior Tree the variable is not being given a default value in the Details box of the Decorator.
I had the same error but I found my issue was different:
I had created a blueprint Decorator which was not using and variables of type Blackboard Key Selector. When I added a dummy variable of this type and made it editable by clicking on the eye icon, my project packaged successfully.