Bug MissingReferenceException: ARAnchor is destroyed but still trying to access it.

Hi,

In my code i delete the ARAnchor Gameobject at a certain time. When I do that I get the following error:
"MissingReferenceException: The object of type 'ARAnchor' has been destroyed but you are still trying to access it."

It happens on line 411 of the ARTrackablemanager class (trackable.transform.SetPositionAndRotation):

Code (CSharp):

1. void SetSessionRelativeData(TTrackable trackable, TSessionRelativeData data)
2.         {
3.             trackable.SetSessionRelativeData(data);
4.             var worldSpacePose = sessionOrigin.trackablesParent.TransformPose(data.pose);
5.             trackable.transform.SetPositionAndRotation(worldSpacePose.position, worldSpacePose.rotation);
6.         }

I believe after a bit of research its here and I think its happening in this method:

Code (CSharp):

1.  TTrackable CreateOrUpdateTrackable(TSessionRelativeData sessionRelativeData)
2.         {
3.             var trackableId = sessionRelativeData.trackableId;
4.             if (m_Trackables.TryGetValue(trackableId, out var trackable))
5.             {
6.                 m_PendingAdds.Remove(trackableId);
7.                 trackable.pending = false;
8.                 SetSessionRelativeData(trackable, sessionRelativeData);
9.             }
10.             else
11.             {
12.                 trackable = CreateTrackable(sessionRelativeData);
13.                 OnCreateTrackable(trackable);
14.             }
15.  
16.             OnAfterSetSessionRelativeData(trackable, sessionRelativeData);
17.             trackable.OnAfterSetSessionRelativeData();
18.             return trackable;
19.         }

Is there something im doing wrong or is this a bug in AR Foundation that can be solved?

Using AR Foundation 4.1.1 and calling the method "Object.Destroy(ARAnchor gameobject)"

Delaying the destroy fixes the problem, but it feels like a hack

 

Do you mean the new way of using Anchors with addcomponent<> and destroy()? Because in my project I directly attach them to the plane with AttachAnchor(). @TreyK-47

 

Maybe related. When using Destroy(anchor.gameObject) as suggested, eventually, at some point the error below gets thrown once for every Anchor that has been destroyed until then.
The old m_AnchorManager.RemoveAnchor(anchor) works with out issues.

MissingReferenceException: The object of type 'ARAnchor' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].SetSessionRelativeData (TTrackable trackable, TSessionRelativeData data) (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:394)
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].CreateOrUpdateTrackable (TSessionRelativeData sessionRelativeData) (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:426)
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].Update () (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:186)

 

Unfortunally i am not able to set up a minimum bare project with proper debugging.
Here is how to recreate the error:

Unity 2021.3.1f1
with AR Core Template
AR Foundation 4.2.2
ARCore XR Plugin Version 4.2.2 (Installed 4.2.3)
Platform to Android

Open SampleScene
Add a anchor prefab to AnchorCreator in the inspector
To AnchorCreator.Awake() add
InvokeRepeating("RemoveAllAnchors", 5, 5);
BuildAndRun

Tap to create anchors
See them deleted every 5 seconds
After a few rounds the deletion stops working

Presumably because of the error i posted above, maybe someone can verify with debugger attached.

MissingReferenceException: The object of type 'ARAnchor' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].SetSessionRelativeData (TTrackable trackable, TSessionRelativeData data) (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:394)
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].CreateOrUpdateTrackable (TSessionRelativeData sessionRelativeData) (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:426)
UnityEngine.XR.ARFoundation.ARTrackableManager`5[TSubsystem,TSubsystemDescriptor,TProvider,TSessionRelativeData,TTrackable].Update () (at Packages/com.unity.xr.arfoundation@4.2.2/Runtime/AR/ARTrackableManager.cs:186)