Crash at UAnimInstance::DisplayDebug() when a montage ends

UDN case : [Link Removed]

We are seeing a crash in UAnimInstance::DisplayDebug() at the following line:

FString MontageEntry = FString::Printf(TEXT("%i) %s CurrSec: %s NextSec: %s W:%.2f DW:%.2f"), MontageIndex, *MontageInstance->Montage->GetName(), *MontageInstance->GetCurrentSection().ToString(), *MontageInstance->GetNextSection().ToString(), MontageInstance->GetWeight(), MontageInstance->GetDesiredWeight());

The crash occurs because MontageInstance->Montage is nullptr.

This appears to happen when a particular character finishes playing an animation montage. The sequence of events appears to be, in Frame N:

• (PrePhysics) In UAnimInstance::ClearMontageInstanceReferences(), the montage instance is removed from the ActiveMontagesMap.
• (PostPhysics) FAnimMontageInstance::Terminate() sets the Montage property from the montage instance to nullptr.
• At the end of Frame N, the crash in UAnimInstance::DisplayDebug() happens because the montage instance is still in the MontageInstances array, but its Montage property is nullptr.

I believe this issue is related to physics animation blending due to the involvement of FParallelBlendPhysicsCompletionTask in the above call stack, plus the fact that if I remove the component that enables that feature on the character then the crash no longer occurs. Unfortunately I have not been able to reproduce this in a clean project.

I have not been able to reproduce, I have fixed the crash by looking at the crash itself and looking at the code