The exact error is:
“BlueprintEditorCompileResults:Error: Error  AnimGraphNode_RotationOffsetBlendSpace_8  references an unknown blend space”
This error emanates from ‘UAnimGraphNode_RotationOffsetBlendSpace::ValidateAnimNodeDuringCompilation’ because ‘Node.BlendSpace’ is NULL.
That property gets set to NULL within this block of code within ‘FAnimBlueprintNodeOptionalPinManager::PostInitNewPin’:
			if (bIsAnimationAsset)
			{
				ObjectProperty->SetObjectPropertyValue(PropertyAddress, nullptr);
			}
Clean sample project attached for immediate repro.
Repro steps:
- 
Open Additive Pin Bug project
 - 
Open “BugRepro” Animation Blueprint
 - 
Note that it is currently in error
 - 
Uncheck ‘as pin’ checkbox for the Blend Space property
 - 
Compile that Animation Blueprint. The error goes away when the node is able to use its internally specified Blend Space that is not exposed as a pin.
 - 
Check it again to expose the pin
 
Result: Compile Error comes back. Feeding a variable that matches the blendspace set in the node makes no difference
Expected: For the node to accept Blend Space pin input without compile error
Our goal is to be able to feed a UAimOffsetBlendSpace1D variable into that pin. This works fine for Blendspace Player using nonadditive offsets, but not for UAnimGraphNode_RotationOffsetBlendSpace.

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