Hmm I tried looking it up and only found other people talking about it happening while importing assets. Just to check, you're using Unity 5.3.2+, right? Maybe it's a version problem?
I'm reading that it might have to do with "Force Text Mode" being enabled under "Asset Serialization" in "Edit > Project Settings > Editor". Try changing that setting to "Mixed"? This is just my best guess.
Hi, 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