com.unity.xr.arsubsystems 3.1.5 Package Notes

Fixed an issue which could throw an exception when subsystems were run in the Editor (e.g., for simulation or remoting). This could happen when a trackable (e.g., a plane or anchor) was removed. This did not affect Player builds (i.e., on device). This is an example of the exception and associated callstack:
```
NullReferenceException: Object reference not set to an instance of an object
Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle.CheckReadAndThrow (Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle handle)
Unity.Collections.NativeArray`1[T].Copy (Unity.Collections.NativeArray`1[T] src, Unity.Collections.NativeArray`1[T] dst)
Unity.Collections.NativeArray`1[T].CopyFrom (Unity.Collections.NativeArray`1[T] array)
UnityEngine.XR.ARSubsystems.TrackableChanges`1[T]..ctor (System.Void* addedPtr, System.Int32 addedCount, System.Void* updatedPtr, System.Int32 updatedCount, System.Void* removedPtr, System.Int32 removedCount, UnityEngine.XR.ARSubsystems.XRReferencePoint defaultT, System.Int32 stride, Unity.Collections.Allocator allocator)
...
```

com.unity.xr.arsubsystems 3.1.5