Hi everyone.
I started to do an Aggressive AI with the Follower Ai set in the documentation.
When it come to follow the player, i get this error :
(i use blackboard)
I see this happen sometimes, sometimes with hot reload, sometimes when another user has made changes to this or a related blueprint. It happens less since 4.7, but it still does happen.
First thing to try “refresh all nodes” from the File menu when you have the blueprint open. If it compiles, save and you should be good.
Second thing to try: check your blueprint parent and make sure you’ve subclassed [FONT=Courier New]BTTask_BlackboardBase and not just [FONT=Courier New]BTTask. The “Get Blacboard Value as xxx” functions are not part of [FONT=Courier New]BTTask, they’re part of [FONT=Courier New]BTTask_BlackboardBase. If you have subclassed [FONT=Courier New]BTTask, simply choose Reparent Blueprint from the File menu when you have the Blueprint open.
If those dont’t work,
Try closing the project, deleting the [FONT=Courier New]/Intermediate, [FONT=Courier New]/Binaries, [FONT=Courier New]/DerivedDataCache (if it exists). If you have C++ in your project, try regenerating your visual studio or Xcode project. Then launch the project again. This will cause it to rebuild various derived files, one of which may have gotten out of sync.
up, still don’t find the answer 
I am in same as you… My AI is still stuck in root… Just start wonder if it is a minon glitch?
I’m having the same issue… I followed the official tutorial here but my behavior tree gets stuck on the root node.
Did you ever sort out the problem?