docs: @UiThread
      public boolean handleBack() {
          ThreadUtils.ensureMainThread();
  
          if (!backstack.isEmpty()) {
              //noinspection ConstantConditions
              if (backstack.peek().controller.handleBack()) {
                  return true;
              } else if (popCurrentController()) {
                  return true;
              }
          }
  
          return false;
      }