Steps to reproduce the issue

Try to edit a template that has been deleted using a bookmark or cached url in your browser.

The easiest way to replicate is to go to edit the templates files

http://127.0.0.1:1025/administrator/index.php?option=com_templates&view=templates

Select administrator

Click Atum Details and Files

gets you a url like this

https://example.com/administrator/index.php?option=com_templates&view=template&id=210&file=aG9tZQ==

Change 210 to lets say 666

Expected result

Handled error message - it does this
No PHP warnings

Actual result

Page of PHP warnings and notices (Excuse my horrid green color haha)

Additional comments