Steps to reproduce:
1. Create new Unity project using Universal 2D Template
2. Import provided “SpriteBoris.unitypackage” Sprite
3. Add it in Hierarchy window and select it
4. In Inspector, add Sprite Skin Component on “Boris” Sprite
5. In Sprite Renderer > Sprite > select Boris
6. In Sprite Skin > Root Bone > select Boris
7. In Bones section add all bones
8. If needed - press “Reset Bind Pose” multiple times in Sprite Skin Component
9. Observe the Editor

Actual results: Editor crash, “System.ArgumentException” errors are spammed in the Console window
Expected results: Editor doesn’t crash, no errors are spammed in the Console window

Reproducible with versions:  6000.4.0a2 (417aa67423b3), 6000.4.4f1, 6000.5.0b4, 6000.6.0a3
Not reproducible with versions: 6000.3.14f1, 6000.4.0a1 (b27b8eafe426)

Tested on (OS): macOS Silicon Tahoe 26.4.1 (M1)

Notes: 

on 6000.4 stream version might require pressing “Reset Bind Pose” button multiple times for Editor to crash, on 6000.5 and 6000.6 - after adding the last bone - Editor usually crashes without pressing “Reset Bind Pose” button
{code:java}
System.ArgumentException: Key: 958874864 is not present.
This Exception was thrown from a job compiled with Burst, which has limited exception support.
#3 Unity.Collections.NativeHashMap`2[[System.Int32, netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51],[UnityEngine.U2D.Animation.TransformAccessJob+TransformData, Unity.2D.Animation.Runtime, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]], Unity.Collections, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null.get_Item(Unity.Collections.NativeHashMap`2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[UnityEngine.U2D.Animation.TransformAccessJob+TransformData, Unity.2D.Animation.Runtime, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]], Unity.Collections, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null this, System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 key) -> UnityEngine.U2D.Animation.TransformAccessJob+TransformData, Unity.2D.Animation.Runtime, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null_831720882744026ebd0bd7aa0f62c853 from Unity.2D.Animation.Runtime, Version=
#4 Unity.Jobs.IJobParallelForExtensions.ParallelForJobStruct`1<UnityEngine.U2D.Animation.BoneDeformBatchedJob>.Execute(ref UnityEngine.U2D.Animation.BoneDeformBatchedJob jobData, System.IntPtr additionalPtr, System.IntPtr bufferRangePatchData, ref Unity.Jobs.LowLevel.Unsafe.JobRanges ranges, int jobIndex) -> void_f05ee1f324267e873e84fcc10645db4b from UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null
#5 6d933222c5b0c0c915d062861958d408
#6 ExecuteJob(ManagedJobData, void ()(void, void, void, void, int), int, unsigned char)
#7 ExecuteJobCopyData(ManagedJobData, void ()(void, void, void, void, int), int)
#8 ujob_execute_job(ujob_control_t, ujob_lane_t, ujob_job_t*, ujob_handle_t, unsigned int)
{code}