This is not the problem, please check the service log. By default the type of the fields you are showing is "Unknown". You can check this using a new installation of Sitefinity 4.2.
There is some issue with ModuleEditor Service that is used when you click on custom field section. The service gets all fields - default and custom one and bind them to the FieldsGrid control you see.
All the best,
the Telerik team
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