JS error when saving if main module is User

If the main module is set to Tools (and not User), when saving constants, a JS error occurs.

Lfeditor.js?bust=1560162151:42 Uncaught TypeError: Cannot set property 'value' of undefined
    at Object.submitLanguageFileEdit (Lfeditor.js?bust=1560162151:42)
    at HTMLButtonElement.onclick (index.php?route=%2Ftools%2FLfeditorLfeditor%2F&token=547afaa4a68e6c29e48d5d9d33a5c6b98334e8ae&tx_lfeditor_tools_lfeditorlfeditor[buttonType]=3&tx_lfeditor_tools_lfeditorlfeditor[action]=editFile&tx_lfeditor_tools_lfeditorlfeditor[controller]=EditFile:114)