Coping news page creates an internal server error

## Describe the bug
A clear and concise description of what the bug is

## URL

## To Reproduce
Steps to reproduce the behavior
example:
1. Go to 'https://joplin-pr-4720-news-copy.herokuapp.com/admin/pages/search/?q=&content_type=news_page.newspage'
2. Click on More and Select copy
3. try to submit the page as is (the required fields are highlighted)
4. Change the required highlighted fields
5. click in copy this page
6. See error (screen shoot)

## Current behavior
it looks like the user is only warned to change the english slug after a failing to change all the fields in order to submit the copied page. If a user changes only changes the english title and the slug it would be redirected to a server error page.

## Expected behavior
After failing to change all the required fields user should have a warning on all the fields that must be altered in order no to get the server error
one more thing that could be done to prevent this is to disabled the "copy this page" button until all the fields are modify

## Screenshots

## Device and browser

## Additional context

I can see this happening specially since most user would only be concerned a about the english page at the moment of copying a page