NullPointerException in the IF statement with two negative conditions in Java + Selenium

I'm just starting to work with Java and Selenium and stuck with getting a NullPointerException in the following if statement when trying to switch to a third window opened. My test has two windows opened already. Each window has its handle saved into a variable.

Then this method LessonHeader.clickLessonPreview(); opens a third window that I need to switch to.

String windowLMS = ApplicationManager.driver.getWindowHandle();
String windowLD = ApplicationManager.driver.getWindowHandle();

 private void previewLesson() {
    WebDriverWait wait = new WebDriverWait(ApplicationManager.driver, 5);
    assert ApplicationManager.driver.getWindowHandles().size() == 2;
    LessonHeader.clickLessonPreview();
    wait.until(numberOfWindowsToBe(3));

    for (String windowHandle : ApplicationManager.driver.getWindowHandles()) {
      if (!windowLMS.contentEquals(windowHandle) && !windowLD.contentEquals(windowHandle)) {
        ApplicationManager.driver.switchTo().window(windowHandle);
        break;
      }
      wait.until(elementToBeSelected(By.id("titlestartbutton")));
    }
  }

The exception happens in this line: if (!windowLMS.contentEquals(windowHandle) && !windowLD.contentEquals(windowHandle)) {