Used in Puppeteer when creating a Browser instance. As such it should be necessary for Gutenberg and our alpha release.

The following is a top error when running the gutenberg tests:

    Protocol error (Target.setDiscoverTargets): this.browsingContext is null get id@chrome://remote/content/targets/TabTarget.jsm:55:5
    onTargetCreated@chrome://remote/content/domains/parent/Target.jsm:72:9
    setDiscoverTargets@chrome://remote/content/domains/parent/Target.jsm:64:12
    execute@chrome://remote/content/domains/Domains.jsm:99:25
    execute@chrome://remote/content/sessions/Session.jsm:54:25
    onPacket@chrome://remote/content/Connection.jsm:210:36
    onMessage@chrome://remote/content/server/WebSocketTransport.jsm:85:18
    handleEvent@chrome://remote/content/server/WebSocketTransport.jsm:67:14
    EventListener.handleEvent*ready@chrome://remote/content/server/WebSocketTransport.jsm:29:17
    Connection@chrome://remote/content/Connection.jsm:33:20
    handle@chrome://remote/content/targets/Target.jsm:56:18
    async*createHandlerFunc/<@chrome://remote/content/server/HTTPD.jsm:2127:13
    handleResponse@chrome://remote/content/server/HTTPD.jsm:2452:36
    process@chrome://remote/content/server/HTTPD.jsm:1347:26
    _handleResponse@chrome://remote/content/server/HTTPD.jsm:1794:22
    _processBody@chrome://remote/content/server/HTTPD.jsm:1639:14
    onInputStreamReady@chrome://remote/content/server/HTTPD.jsm:1518:14

      at ../../node_modules/puppeteer/lib/Connection.js:74:56
      at Connection.send (../../node_modules/puppeteer/lib/Connection.js:73:12)
      at Function.create (../../node_modules/puppeteer/lib/Browser.js:34:22)
      at FirefoxLauncher.connect (../../node_modules/puppeteer/lib/Launcher.js:451:20)
          at runMicrotasks (<anonymous>)
      at Object.resetBrowser (../../node_modules/jest-environment-puppeteer/lib/PuppeteerEnvironment.js:119:31)
      at PuppeteerEnvironment.setup (../../node_modules/jest-environment-puppeteer/lib/PuppeteerEnvironment.js:140:5)

Maja, this most likely happens when the method gets called early enough right after startup and not all browsers have been loaded in any of the tabs. Could you please check how many tabs are getting opened? If that's more than one, please try some of the Marionette/geckodriver preferences so that only a single tab is opened during startup, and as best with about:blank. Does that help?

In any way it would be good to file it as a separate bug and mark it blocking this meta bug.

The error from Comment 2 doesn't come up anymore.