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First of all, great asset!
Just to add a bit more precision, this does indeed happen when you import the asset with the serialization setting set to 'ForceText'. Changing it to 'Mixed' with the asset already imported will get rid of the errors but all builtin content (Gradients, etc.) won't work. The trick is to set the serialization setting to 'Mixed' before importing the asset and then set it back to 'Force Text'. Then all the builtin content will work.

Just thought I'd save everyone the 5 mins it takes to figure it out. :P

Btw it happened to me on 5.5.0f3.