Cycle testdriver loop correctly

Otherwise, the executor, which expects a list, will fail to unpack
`None`.

tools/wptrunner/wptrunner/executors/executorwebdriver.py

@@ -614,7 +614,7 @@ def do_testharness(self, protocol, url, timeout):
             # cases where the browser crashes during script execution:
             #
             # https://github.com/w3c/webdriver/issues/1308
-            if not isinstance(result, list) or len(result) != 2:
+            if not isinstance(result, list) or len(result) != 3:
                 try:
                     is_alive = self.is_alive()
                 except error.WebDriverException:
@@ -623,6 +623,16 @@ def do_testharness(self, protocol, url, timeout):
                 if not is_alive:
                     raise Exception("Browser crashed during script execution.")
 
+            # A user prompt created after starting execution of the resume
+            # script will resolve the script with `null` [1, 2]. In that case,
+            # cycle this event loop and handle the prompt the next time the
+            # resume script executes.
+            #
+            # [1]: Step 5.3 of https://www.w3.org/TR/webdriver/#execute-async-script
+            # [2]: https://www.w3.org/TR/webdriver/#dfn-execute-a-function-body
+            if result is None:
+                continue
+
             done, rv = handler(result)
             if done:
                 break