问this.handleError不是一个函数

EN

get(relativeUrl: string, _httpOptions?: any) {

    var response = this.http.get(baseUrl + relativeUrl, _httpOptions)
          .pipe(
          catchError(this.HandleRequiredErrorResponse)
        ).toPromise();

            var responsedata = new ResponseData;

            responsedata.statusCode = '200';
            responsedata.response = response;

            return responsedata;
}

HandleRequiredErrorResponse(errorReponse: HttpErrorResponse) {
    var response = new ResponseData;

    if (errorReponse.status == 400) {
      response.statusCode = errorReponse.status.toString();
      response.response = errorReponse.error;
    }
    else if (errorReponse.status == 404) {
      response.statusCode = errorReponse.status.toString();
      response.response = errorReponse.error;
    }
    else {
      console.log(errorReponse)
      let err = this.handleError(errorReponse);//handleError is undefined

      return err;
    }

    return new ErrorObservable(response);
  }

handleError(error: HttpErrorResponse) {
    if (error.error instanceof ErrorEvent) {
      console.error('An error occurred:', error.error.message);
    } else {
      console.error(
        `Backend returned code ${error.status}, ` +
        `body was: ${error.error}`);
    }
    return new ErrorObservable(
      'Something bad happened; please try again later.');
  }

import { HttpClient, HttpErrorResponse } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { ErrorObservable } from 'rxjs/observable/ErrorObservable';
import { catchError } from 'rxjs/operators';