Seen in production log:
[2019-03-25 05:41:52] php.WARNING: Notice: Undefined index: fallback {"exception":"[object] (ErrorException(code: 0): Notice: Undefined index: fallback at /mnt/nfs/labstore-secondary-tools-project/svgtranslate/app/src/OOUI/TranslationsFieldset.php:31)"} []