Import backup: Uncaught (in promise) TypeError: installedIds.has is not a function

[arantius]

When importing a Greasemonkey backup, I got the error Uncaught (in promise) TypeError: installedIds.has is not a function at:

greasemonkey/src/content/backup/import.js

Line 99 in 5e17a8b

if (gImportOptions.replace || !installedIds.has(userScript.id)) {

In version 4.12.0.

[arantius]

This only happens when un-checking the (checked by default) "overwrite already installed scripts ..." option.