Chaptering doesn't work if no VideoTextTrackChapters field

[GauthierGitHub]

Dear Alchemy team, We faced a problem using video tools. We wasn't able to create chapter on videos. After investigation, it's because we didn't set a "VideoTextTrackChapters" field in this database. But there was no error message (or any message) about it when we was using it, we had to search into the code to find it. Could you improve UI by adding a message when this field is not set ? Best Regards, Gauthier