I'm using jsunit to test enigmail and when i start up thunderbird using
jsunit with a simple test, it terminates with a strange warning.

(i'm omitting warnings from wetransfer, which happen at startup, and are
covered in #944208):

here's the setup:

    x=$(mktemp -d profdir.XXXXXX)
    touch main.js
    cat > "$x/prefs.js" <<EOF
    user_pref("browser.dom.window.dump.enabled", true);
    user_pref("extensions.autoDisableScopes", 14);
    EOF
    mkdir -p "$x/extensions"
    printf /usr/share/xul-ext/jsunit/ > "$x/extensions/jsunit@enigmail.net"
    thunderbird --headless --profile "$(pwd)/$x"

and the output is:

    JSUnit: startup
    JSUnit starting tests
    Starting JS unit tests main.js


    FINAL STATS

    TestResult: executed : 0
    TestResult: succeeded: 0
    TestResult: failed   : 0
    JavaScript error: chrome://messenger/content/mailWindow.js, line 226: TypeError: messagepane.docShell is null


This happens even if i don't use --headless.

It's possible that the error resides in something jsunit is doing, but
i'm not sure of that, or how i would debug it.