Changing the importer from "NoesisFontImporter" to "UnityEditor.TrueTypeFontImporter (Default)" should work. It seems the error is coming from Unity because they are still trying to render a thumbnail. I will have a look at this ASAP.

Does restarting Unity fix the issue?

I already tried restarting Unity and deleting .meta files but that doesn't work.

I also did the test on a new project and the same thing happens.

The following patch fixes this issue

Index: NoesisPostprocessor.cs

--- NoesisPostprocessor.cs (revision 11128)
+++ NoesisPostprocessor.cs (working copy)
@@ -99,7 +99,11 @@
else if (IsFont(assetPath))
{
// Noesis uses a custom font importer that replaces Unity's default one

• AssetDatabase.SetImporterOverride<NoesisFontImporter>(assetPath);
• // If there is not override yet, change the importer to Noesis
• if (AssetDatabase.GetImporterOverride(assetPath) == null)
• {
• AssetDatabase.SetImporterOverride<NoesisFontImporter>(assetPath);
• }
  }
  }
  }