Invalid field content lost when selecting different entry

[lenhard]

JabRef version 3.5 onwards.

As first mentioned in #1863 (comment)

Steps to reproduce:

1. Insert {a in article's title field
2. Select different bib entry
3. Return to first entry, field is reset to prior value

Although the field value is invalid. It should not simply be deleted without a message. Instead, it should still be present and, possibly, the field should be colored red. Alternatively, an error message could pop up, forcing the user to return to the corrupt field.

[lenhard]

Fixed by #1893

[lenhard]

Correcting: Though indicated by the PRs, the issue is not really solved.

[lenhard]

This seems to be solved in current master.