Error message in AI Behavior Tree

Hi, I’ve been trying to get familiar with UE4’s Behavior Tree but I keep getting this message whenever I have a Blackboard Decorator, and it wont finish the Tasks. It does go through without a decorator.

I’m using 4.21 with the Paragon Morigesh BP. I have no idea what this means.

Can anyone help explaining this to me?

268344-ai-behaviortree-error.png