Troubleshooting Blackboard and RunSubgraph issues

Resolve common issues related to Blackboard variables and RunSubgraph nodes in Unity Behavior.

These issues include incorrect default values, stack overflow errors, and null SharedBlackboardVariable values in Behavior agents.

Issue 1: RunSubgraph node default value doesn't update

Symptoms

When you change a subgraph’s Blackboard Variable (BBV) default value, the RunSubgraph field keeps the previous value instead of updating. The Inspector shows the outdated value as an override.

Cause

The field stores its current value as an override rather than applying the new Blackboard default.

Resolution

Revert the RunSubgraph field to its default value.

Issue 2: Stack-overflow error with SharedBlackboardVariable

Symptoms

A stack overflow error occurs in Play mode or at runtime. Unity logs reference SharedBlackboardVariable or RuntimeBlackboardAsset.

Cause

A crash in the Behavior Editor can serialize invalid SharedBlackboardVariable data in the RuntimeBlackboardAsset.

Resolution

Upgrade to Unity Behavior version 1.0.3 or later.

Check your original Blackboards to ensure no SharedBlackboardVariable is serialized in the RuntimeBlackboardAsset.

Issue 3: SharedBlackboardVariable default value is null

Symptom

A SharedBlackboardVariable on Behavior(Graph)Agent shows a null default value. This issue might appear after you fix a previous stack overflow problem.

Causes

The SharedBlackboardVariable was serialized in RuntimeBlackboardAsset with an invalid default value.

Resolution

Perform the following checks to resolve this issue:

  • Update to Unity Behavior version 1.0.3 or later.

  • Check that no SharedBlackboardVariable appear under Blackboard > Variables in RuntimeBlackboardAsset.

  • To refresh serialization, do the following:

    • Open the faulty behavior graph.
    • Add a new Blackboard variable.
    • Save the graph to refresh the asset.
    • Delete the new variable.
    • Save the graph again.

Additional resources