When you try to create a page without a title, an error message is displayed:

However, this error message persists even if you have entered a title properly, then press Preview.

Steps to reproduce

1. Create a page, enter content but no title
2. Press Save (error message displayed as expected)
3. Enter any title
4. Press Preview (the error message persists)
5. Press Edit (the error is still there)

Expected behavior: the error should disappear once it is not relevant anymore.