Additive animation `both writing Animation_source_clip at the same time`

Release Type: Official Release

Version: 4.0.1.1428

Platform(s): Windows

Describe the bug
When attempting to create an additive animation based on the instructions provided in the docs, the animations fail to load due to a FBX import error.

To Reproduce
Steps to reproduce the behavior:

1. In the asset view, click Add asset and select animation.
2. When the file chooser window opens click cancel.
3. When the popup shows asking if you want to create the Animation without a source file, click Create.
4. Choose an FBX animation file for the Source field.
5. Set the Type field to Difference Clip.
6. Choose a FBX animation file for the Reference field.
7. Choose a skeleton using the asset selector for the Skeleton field

Expected behavior
The asset should be useable the same as any other animation clip asset.

Screenshots

Log and callstacks
AssetBuilder log in debug window. Shows the both writing Animation_source_clip at the same time error that is displayed in the Asset errors tab:
AssetBuilder.txt

Additional context
This can be replicated on an completely new project.
The full game solution that should have the asset errors. Imported resources located in the zip under the folder Resources.
MyGame.zip

Start with the additive-animation documentation and reproduce the issue using the supplied MyGame.zip and FBX files. Review AssetBuilder.txt for the “both writing Animation_source_clip at the same time” failure and follow the asset-view steps to isolate the import path. Done means the additive animation loads and is usable like another animation clip asset.