When editing an existing content object from the frontend, with legacy_mode: false, if we hit the preview button in Website Toolbar we won't be able to edit the content again, due to a version edit conflict.

Steps to reproduce:

1. Log in to an eZ Demo based frontend, in order to create content
2. Start creating a new article, fill it with some content, and publish it
3. Re-edit the article you just created
4. Hit the preview button in Website Toolbar (you will enter versionview mode)
5. Now hit the edit button to go back to the edit mode

You will get a version edit conflict.