title: A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object
body: <p>I'm just new in ReactJS and I have a problem. I can't solve it. It seems everything is all right, but still console puts me: </p>
  
  <blockquote>
    <p>A valid React element (or null) must be returned. You may have
    returned undefined, an array or some other invalid object.</p>
  </blockquote>
  
  <p>Here's my code: </p>
  
  <pre><code>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 =&gt; res.json())
      .then(res =&gt;{
        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 =
        &lt;div className="pokemon--species--list"&gt;
          {species.map((pokemon,index) =&gt; &lt;Pokemon key={pokemon.name} id={index+1} pokemon={pokemon}/&gt;)}
        &lt;/div&gt;;
      }
      else if(loading &amp;&amp; !fetched){
          content = &lt;p&gt; Loading ...&lt;/p&gt;;
      }
      else{
        content = &lt;div/&gt;;
      }
      return (
        &lt;div&gt;
          {content}
        &lt;/div&gt;
      );
    }
  }
  
  export default PokemonList;
  </code></pre>
  
  <p>Pokemon.js</p>
  
  <pre><code>import React from 'react';
  import ReactDOM from 'react-dom';
  
  
  class Pokemon extends React.Component {
    render() {
      const {pokemon, id} = this.props;
      return
        &lt;div className="pokemon--spacies"&gt;
          &lt;div className="pokemon--spacies--container"&gt;
            &lt;div className="pokemon--spacies--sprite"&gt;
              &lt;img src={`/public/sprites/${id}.png`} /&gt;
            &lt;/div&gt;
            &lt;div className="pokemon--spacies--name"&gt; {pokemon.name }&lt;/div&gt;
          &lt;/div&gt;
        &lt;/div&gt;;
    }
  }
  
  export default Pokemon;
  </code></pre>
  
  <p>Thanks for help!</p>