I am working on my NPC's ability to detect player within a range. But when I directly refer it in the generated action script, I get a null pointer. I have double confirmed that the player is not null. If I just use GameObject.FindWithTag("Player"), it will work.

I wish to the action class use the function provided by detector class to get pointer to the player. I am using the unity 6 behaviour package.

My Detector class

void Start()
{
    if (player == null)
    {
        player = GameObject.FindWithTag("Player");
    }

    distance = Vector3.Distance(transform.position,player.transform.position);
}

public GameObject player; // Assign this in the Unity Editor
private float detectionRadius = 100f; // Detection radius
private float distance = 0f;

void Update()
{
    if (player != null)
    {
        distance = Vector3.Distance(transform.position, player.transform.position);
        //Debug.Log($"Distance to player: {distance}");
    }
}

public Boolean ifWithinDetectDistance()
{
    return distance > detectionRadius ? false : true;
}

Generated action class

[NodeDescription(name: "PlayerDetector", story: "Check if [detector] has a [target]"]
public partial class PlayerDetectorAction : Action
{
    [SerializeReference] public BlackboardVariable<TargetDetector> Detector;
    [SerializeReference] public BlackboardVariable<GameObject> Target;
    protected override Status OnStart()
    {
        if (Detector.Value.ifWithinDetectDistance())
        {
             Target.Value = Detector.Value.player;

             if (Target.Value == null)
                 Debug.Log($"player is null");

             return Status.Success;
        }

        return Status.Failure;
    }
}

I am working on my NPC's ability to detect player within a range. But when I directly refer it in the generated action script, I get a null pointer. I have double confirmed that the player is not null. If I just use GameObject.FindWithTag("Player"), it will work.

I wish to the action class use the function provided by detector class to get pointer to the player. I am using the unity 6 behaviour package.

My Detector class

void Start()
{
    if (player == null)
    {
        player = GameObject.FindWithTag("Player");
    }

    distance = Vector3.Distance(transform.position,player.transform.position);
}

public GameObject player; // Assign this in the Unity Editor
private float detectionRadius = 100f; // Detection radius
private float distance = 0f;

void Update()
{
    if (player != null)
    {
        distance = Vector3.Distance(transform.position, player.transform.position);
        //Debug.Log($"Distance to player: {distance}");
    }
}

public Boolean ifWithinDetectDistance()
{
    return distance > detectionRadius ? false : true;
}

Generated action class

[NodeDescription(name: "PlayerDetector", story: "Check if [detector] has a [target]"]
public partial class PlayerDetectorAction : Action
{
    [SerializeReference] public BlackboardVariable<TargetDetector> Detector;
    [SerializeReference] public BlackboardVariable<GameObject> Target;
    protected override Status OnStart()
    {
        if (Detector.Value.ifWithinDetectDistance())
        {
             Target.Value = Detector.Value.player;

             if (Target.Value == null)
                 Debug.Log($"player is null");

             return Status.Success;
        }

        return Status.Failure;
    }
}

• c#
• unity-game-engine
• null

1 Answer 1

1. Your PlayerDetectorAction uses BlackboardVariable<TargetDetector> Detector;. This means the action expects to find a reference to a TargetDetector component stored in the Behavior Tree's Blackboard.

2. The NullReferenceException occurs because Detector.Value is null. This implies that the Blackboard variable linked to the Detector field in your action node has not been correctly assigned the instance of the TargetDetector component running on your NPC.

3. The action script itself doesn't automatically know which TargetDetector instance in your scene it should be talking to. You need to explicitly tell it via the Blackboard.

   1. In the Unity Editor, select the GameObject that has the Behavior Tree component.

   2. Inspect the Behavior Tree component. You should see a section for the Blackboard.

   3. Ensure you have a Blackboard variable defined. You might need to add one. Let's say you call it MyTargetDetector. The type of this variable should be Object or, if the system allows, specifically TargetDetector.

   4. Assign the reference: Find the TargetDetector component on your NPC GameObject (either the same GameObject running the tree or another one). Drag the TargetDetector component from the NPC's Inspector into the value field of the MyTargetDetector Blackboard variable.

   5. Select the PlayerDetector node within your Behavior Tree graph editor.

   6. In the Inspector for the PlayerDetector node, find the Detector field (which corresponds to your BlackboardVariable<TargetDetector> Detector;).

   7. Link this field to the Blackboard variable: There should be a dropdown or selection mechanism. Choose the MyTargetDetector variable you created and assigned in the Blackboard. Make sure it's not set to "(None)".