With an RTL language active, an undefined index error is thrown on a text format configuration form, naming ckeditor as source

Problem/Motivation

While testing #2018913: Vertical tab styling for tabs should not be applied when JavaScript is disabled and no tabs are present, we ran into this error on the Full HTML text format configuration screen, while viewing the page with the Hebrew language installed and active (by prepending the path with /he)

The error reads Notice: Undefined index: image_alternative_rtl in {closure}(). (line 58 of core/modules/ckeditor/ckeditor.admin.inc)

Proposed resolution

None yet.

Remaining tasks

Investigate the error.