Apologies if this has already been resolved – I found this issue as I had the same thing when trying to create some enemy patrol behaviour.
Blueprint Runtime Error: “Accessed None trying to read property CallFunc_GetBlackboard_ReturnValue”. Node: Set Value as Object Graph: UpdatePatrolPointBB Function: Execute Ubergraph BP Enemy Blueprint: BP_Enemy
I’d forgotten to change the Pawn AI Controller Class to be my BP_EnemyController on the instance of the blueprint that I had added into the level. Once this was done, the blueprints worked as expected.
It might be a little late for you, but could save the next person that misses this step.
Cheers
John