Handling “Selenium (WebDriver) Exception: this.getWindow() is null”

I was trying to create a simple script to do few things :

• Launch Twitter
• Click on “Sign in”
• Log in with username "testmail" and password "password"

Relevant Webdriver script is:

public void launchTwitter() throws Exception {
        browser.get("http://www.twitter.com");
    }
public void clickOn(String string1) throws Exception {
        browser.findElements(By.className("promotional")).get(0).submit();
    }
    public void logInWithUsernameAndPassword(String string1, String string2)
    throws Exception {
        browser.findElement(By.id("username")).sendKeys(string1);
        browser.findElement(By.id("password")).sendKeys(string2);
        browser.findElements(By.className("submit")).get(1).submit();
    }

While trying to play it back on Firefox 4, script failed on below step

browser.findElement(By.id("username")).sendKeys(string1);

and stack trace looked like:

org.openqa.selenium.WebDriverException: this.getWindow() is null
System info: os.name: 'Windows 7', os.arch: 'x86', os.version: '6.1', java.version: '1.6.0_24'
Driver info: driver.version: RemoteWebDriver
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at org.openqa.selenium.remote.ErrorHandler.createThrowable(ErrorHandler.java:131)
    at org.openqa.selenium.remote.ErrorHandler.throwIfResponseFailed(ErrorHandler.java:105)
    at org.openqa.selenium.remote.RemoteWebDriver.execute(RemoteWebDriver.java:409)
    at org.openqa.selenium.remote.RemoteWebDriver.findElements(RemoteWebDriver.java:203)
    at org.openqa.selenium.remote.RemoteWebDriver.findElementsByLinkText(RemoteWebDriver.java:221)
    at org.openqa.selenium.By$ByLinkText.findElements(By.java:218)
    at org.openqa.selenium.remote.RemoteWebDriver.findElements(RemoteWebDriver.java:180)
    at Twitter.goBack(Twitter.java:20)
Caused by: org.openqa.selenium.remote.ErrorHandler$UnknownServerException: this.getWindow() is null
System info: os.name: 'Windows 7', os.arch: 'x86', os.version: '6.1', java.version: '1.6.0_24'

It’s strange! So Google comes to rescue at this point of time. After searching I got that there are lot many others who are facing this issue.

Refer this for more detail: http://code.google.com/p/selenium/issues/detail?id=1438 

If you don’t want to get re-directed I will post the quick solution, try

browser.switch_to.default_content

As per Simon, this is probably an issue with Firefox and not Selenium. 

Just to note that this works perfectly right with Firefox 3.6.