code: public void selectMainWindow() {
          String action = "Switching to main window";
          String expected = "Main window is selected";
          try {
              driver.switchTo().defaultContent();
          } catch (Exception e) {
              reporter.fail(action, expected, "Main window was not selected. " + e.getMessage());
              log.warn(e);
              return;
          }
          reporter.pass(action, expected, expected);
      }
docs: Select the main window. Used for returning to the main content after
  selecting a frame. If there are nested frames, the main content will be
  selected, not the next frame in the parent child relationship