Do not look for JS errors when the browser session hasn't started

behat_javascript.behat.inc

@@ -88,7 +88,7 @@ class BehatJavascriptContext extends DrupalSubContextBase {
    * @AfterStep
    */
   public function lookForJavascriptErrors(AfterStepScope $scope) {
-    if (!$this->lookForJavascriptErrors) {
+    if (!$this->lookForJavascriptErrors || !$this->getSession()->isStarted()) {
       return;
     }