Never mind. I was able to reproduce what you are seeing. It happens in base 5.14 if you right-click on the template line to open the template editor. The Show Details button shows something like
Code:
calibre, version 5.14.0
ERROR: Unhandled exception: <b>AttributeError</b>:'TemplateDialog' object has no attribute 'mi'
[...]
Python 3.8.5
[...]
Interface language: None
Traceback (most recent call last):
File "calibre\gui2\preferences\save_template.py", line 54, in do_open_editor
File "calibre\gui2\dialogs\template_dialog.py", line 313, in __init__
AttributeError: 'TemplateDialog' object has no attribute 'mi'
The problem is fixed (for me) in the next calibre release.