Reproduction steps:
1. Open the user's attached "Animancer Timeline Preview Crash.zip" project
2. Load the "Platformer" scene
3. Select the "Brain and Actions" GameObject (the "Knight" GameObject's child object) in the Hierarchy window
4. Click on the "Preview this transition" in the Inspector window > "Platformer - Timeline State Component" (a small eye icon on the right to "Animation" variable")
5. Click on the "Play Transition" button in the Timeline Preview window
--Observe the crash

Reproduces on: 2018.4.28f1, 2019.4.13f1, 2020.1.10f1, 2020.2.0b8, 2021.1.0a1

First lines of the stack trace:
0x00007ff7000bb21a (Unity) AnimationLayerMixerPlayable::ProcessRootMotion
0x00007ff7000b1066 (Unity) AnimationPlayable::ProcessRootMotion
0x00007ff7000bb2ba (Unity) AnimationMotionXToDeltaPlayable::ProcessRootMotion
0x00007ff7000a4abe (Unity) AnimationMixerPlayable::MixerProcess
0x00007ff7000a73fb (Unity) AnimationMixerPlayable::ProcessRootMotion
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