Error Unknown Cook Failure (UE 4.12)
It seems like this Ensure is blocking the process.
Ensure condition failed: AttachmentRules.LocationRule == EAttachmentRule::KeepRelative && AttachmentRules.RotationRule == EAttachmentRule::KeepRelative && AttachmentRules.ScaleRule == EAttachmentRule::KeepRelative [File:D:\Build\++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Engine\Private\Components\SceneComponent.cpp] [Line: 1482] AttachToComponent when called from a constructor is only setting up attachment and will always be treated as KeepRelative. Consider calling SetupAttachment directly instead.
It seems that you're using AttachToComponent in one of your constructors. The syntax for doing attachments was changed in 4.12. Here's an excerpt from the release notes:
"Instead of direct reads of USceneComponent::AttachParent, AttachSocketName, and AttachChildren you should use GetAttachParent, GetAttachSocketName, or GetAttachChildren. Direct writes should be replaced with SetupAttachment from constructors or AttachTo once components have been registered."
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