Organization settings crashes for users without access to private stream set for New stream notifications

Log in as an administrator and set ⚙️ → Organization settings → Notifications → New stream notifications (or New user notifications) to a private stream. Then log in as a normal user who doesn’t have access to that private stream, and try to open ⚙️ → Organization settings. The tab fails to load and gives this error:

Uncaught TypeError: can't access property "name", item is undefined
    render dropdown_list_widget.js:39
    setup dropdown_list_widget.js:119
    DropdownListWidget dropdown_list_widget.js:132
    init_dropdown_widgets settings_org.js:613
    build_page settings_org.js:651
    set_up settings_org.js:539
    load_settings_section settings_sections.js:68
    activate_section_or_default settings_panel_menu.js:112
    launch admin.js:147
    do_hashchange_overlay hashchange.js:216
    hashchanged hashchange.js:255
    initialize hashchange.js:308

[timabbott]

We definitely want to allow private streams here; especially for the “signups notifications stream”, there’s a good use case for the feature. And in any case, we need to support the stream being unknown, since guest users may not have access to the notifications stream.

[nishantwrp]

@timabbott ptal at the PR (#17268).