docs: public ErrorPage getErrorPageTraverseRootCause(Throwable th) {
          while (th != null && th instanceof ServletException) { // defect 155880
              // - Check
              // rootcause !=
              // null
              Throwable rootCause = ((ServletException) th).getRootCause();
              if (rootCause == null) {
                  break;
              }
              ErrorPage er = getErrorPageByExceptionType(th);
              if (er != null)
                  return er;
  
              th = rootCause;
          }
          if (th != null)
              return getErrorPageByExceptionType(th);
          return null;
      }