code: public DrawerBuilder withRootView(@IdRes int rootViewRes) {
          if (mActivity == null) {
              throw new RuntimeException("please pass an activity first to use this call");
          }
  
          return withRootView((ViewGroup) mActivity.findViewById(rootViewRes));
      }
docs: Sets the rootView which will host the DrawerLayout
  The content of this view will be extracted and added as the new content inside the drawerLayout
  
  @param rootViewRes the id of a view which will get switched out by the DrawerLayout and added as its child