NullReferenceException error is thrown when creating a Ragdoll

How to reproduce:
1. Open the “Ragdol_test.zip“ project
2. Open the “SampleScene”
3. Right-click in the Hierarchy, then press 3D Object → Ragdoll
4. In the opened window, assign the “unitychan“ GameObject to the Animator field, then press the “Auto-Fill“ button
5. Press the “Create“ button
6. Observe the Console

Expected result: No errors are thrown in the Console
Actual result: A “NullReferenceException“ error is thrown in the Console

Reproducible in: 6000.0.50f1, 6000.0.54f1, 6000.1.13f1, 6000.2.0b11, 6000.3.0a3
Not reproducible in: 6000.0.49f1

Reproduced on: Windows 11 Pro (24H2)
Not reproduced on: No other environment tested

Notes:

• The first line of the error reads: “NullReferenceException: Object reference not set to an instance of an object”
• When “Auto-fill“ is pressed, the correct bone transforms are assigned in the “Advanced Bone Setup“, so this error is only visual (and regards the Ragdoll Builder window)
• The Character Joints are created successfully (at least they show up in the Inspector)
• In 6000.0.49f1 and previous versions, the Auto-Fill functionality was not introduced yet