我只是ReactJS的新手，所以有一个问题。我解决不了 似乎一切都很好，但是控制台仍然让我：

必须返回有效的React元素（或null）。您可能返回了undefined，数组或其他无效对象。

这是我的代码：

import React from 'react';
import ReactDOM from 'react-dom';
import fetch from 'isomorphic-fetch';
import Pokemon from './Pokemon';

class PokemonList extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      species: [],
      fetched: false,
      loading: false,
    };
  }
  componentWillMount(){
    this.setState({
      loading : true
    });
    fetch('http://pokeapi.co/api/v2/pokemon?limit=151').then(res => res.json())
    .then(res =>{
      this.setState({
        species : res.results,
        loading : true,
        fetched : true
      });
    });
  }
  render() {
    const {fetched, loading, species} = this.state;
    let content;
    //This if seems to be the problem
    if(fetched){
      content =
      <div className="pokemon--species--list">
        {species.map((pokemon,index) => <Pokemon key={pokemon.name} id={index+1} pokemon={pokemon}/>)}
      </div>;
    }
    else if(loading && !fetched){
        content = <p> Loading ...</p>;
    }
    else{
      content = <div/>;
    }
    return (
      <div>
        {content}
      </div>
    );
  }
}

export default PokemonList;

宠物小精灵

import React from 'react';
import ReactDOM from 'react-dom';


class Pokemon extends React.Component {
  render() {
    const {pokemon, id} = this.props;
    return
      <div className="pokemon--spacies">
        <div className="pokemon--spacies--container">
          <div className="pokemon--spacies--sprite">
            <img src={`/public/sprites/${id}.png`} />
          </div>
          <div className="pokemon--spacies--name"> {pokemon.name }</div>
        </div>
      </div>;
  }
}

export default Pokemon;

感谢帮助！