I have checked this file:
I then verified it had this setting:
<viewSettings hostType="Telerik.Cms.Engine.WebControls.HtmlContentEditor" layoutTemplatePath="~/Sitefinity/Admin/ControlTemplates/Generic_Content/EditorTemplate.ascx" />
No matter what I change about that file, it doesn't show up in the page/window where you edit Generic Content.
I then went and deleted this file entirely:
Yet, I can somehow still edit Generic Content without getting an error. Clearly, that user control is not what is used when editing Generic Content. Besides the fact that I can delete the user control, that user control doesn't have a TabStrip control in it - which is clearly used by the Generic Content edit page (You see "Basic" and "Advanced" tabs).