Crash on AnimationPlayableOutputExtensions_CUSTOM_InternalSetAnimationStreamSource when passing a default output handle to SetAnimationStreamSource

How to reproduce:

1. Open the attached “IN-151385“ project
2. In the menu bar, select “Bug Repro”->”Set Animation Stream Source with Default Output Handle”
3. Observe the crash

Reproducible with: 6000.0.0f1, 6000.0.82f1, 6000.3.23f1, 6000.5.10f1, 6000.6.0b10, 6000.7.0a5

Reproducible on: Ubuntu 24.04, MacOS 26.5 (M1)
Not reproducible on: No other environments tested

First few lines of the stack trace:
#0 0x007f7362a45330 in (Unknown)
#1 0x00567cd0f6b64d in AnimationPlayableOutputExtensions_CUSTOM_InternalSetAnimationStreamSource(HPlayableOutput const&, AnimationStreamSource)
#2 0x00000041ddf924 in (wrapper managed-to-native) UnityEngine.Experimental.Animations.AnimationPlayableOutputExtensions:InternalSetAnimationStreamSource_Injected (UnityEngine.Playables.PlayableOutputHandle&,UnityEngine.Experimental.Animations.AnimationStreamSource)
#3 0x00000041ddf89f in UnityEngine.Experimental.Animations.AnimationPlayableOutputExtensions:InternalSetAnimationStreamSource (UnityEngine.Playables.PlayableOutputHandle,UnityEngine.Experimental.Animations.AnimationStreamSource)
#4 0x00000041ddf86b in UnityEngine.Experimental.Animations.AnimationPlayableOutputExtensions:SetAnimationStreamSource (UnityEngine.Animations.AnimationPlayableOutput,UnityEngine.Experimental.Animations.AnimationStreamSource