Hi Peter,
If you were in the "Email marketing" module when you created the UI element, you must have selected a different module when you created it. It would have had to be a module that was is in custom development mode.
![Select another module](https://i.ibb.co/F71m2p9/Select-Another-Module.png)
To delete the UI element, navigate to it through the module you selected. For example, if you selected "Mike's Test Module", navigate to
Modules > Mike's Test Module > User Interface
Instead of
Modules > Email Marketing > User Interface
After that, you should be able to select the element in the tree, and then click the delete button.
![Delete a custom UI element](https://i.ibb.co/HC8rygn/Delete-Custom-Ui-Element.png)
Mike