Instead of adding emailtemplate_newedit, add the email overview to the navigation. That page will have a button to go to EmailTemplate_NewEdit.
Using following module: https://marketplace.mendix.com/link/component/2461
There are ‘Use Me’ pages you need to use for proper implementation for a library.
Set your navigation towards “EmailConfiguration” and there should be Template NewEdit pages in there.
I’ll attach an image for your reference.