title: <Menu> throws "type is invalid" error ... no matter what I do
body: <p>I'm attempting to put together a generalized select menu using Material-UI's (1.0.0-beta.4) <code>&lt;Button&gt;</code>, <code>&lt;Menu&gt;</code>, and <code>&lt;MenuItem&gt;</code> components. </p>
  
  <p>Inevitably it blows up on the use of <code>&lt;Menu&gt;</code> throwing:<br>
  <code>React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.</code></p>
  
  <p>As you can see, despite the error message, I'm clearly exporting the class. The error goes away if I remove the <code>&lt;Menu&gt;</code>, even if I leave the child <code>&lt;MenuItem&gt;</code> tags.</p>
  
  <p><strong>Any idea what's going wrong? How to fix this??</strong></p>
  
  <p>Here is the class:</p>
  
  <pre><code>class SelectMenu extends React.Component{
  
      constructor(props){
          super(props);
          this.state = {
              open: false,
              selectedIndex: 0
          };
          this.closeMenu = this.closeMenu.bind(this);
          this.openMenu = this.openMenu.bind(this);
          this.handleSelect = this.handleSelect.bind(this);
      }
  
      openMenu(event){
          this.setState({ open: true, anchorEl: event.currentTarget });
      }
  
      closeMenu(){
          this.setState({ open: false });
      }
  
      handleSelect(selectedValue, i){
          this.props.handleSelect(selectedValue);
          this.state.selectedIndex = i;
          this.closeMenu();
      };
  
      render(){ return (
          &lt;div ref={el=&gt;this.anchorEl=el}&gt;
              &lt;Button
                onClick={this.openMenu}
              &gt;
              {this.props.children}
              &lt;/Button&gt;
              &lt;Menu
                id="simple-menu"
                anchorEl={this.props.anchorEl}
                open={this.state.open}
                onRequestClose={this.closeMenu}
              &gt;
              {this.props.items.map((item,i) =&gt; 
                  &lt;MenuItem 
                      key={i}
                      selected={i === this.state.selectedIndex}
                      onClick={()=&gt;(this.handleSelect(this.props.valueFor(item)))}
                  &gt;
                      {this.props.labelFor(item)}
                  &lt;/MenuItem&gt;
              )}
              &lt;/Menu&gt;
          &lt;/div&gt;
      )}
  }
  
  export default SelectMenu
  </code></pre>
  
  <p>Here's how I'm attempting to use it:</p>
  
  <pre><code>            &lt;SelectMenu
                  id="characterMenu"
                  items={characters.array()}
                  valueFor={((character)=&gt;character.id)} 
                  labelFor={((character)=&gt;character.name)}
                  handleSelect={(selectedValue)=&gt;this.setState({
                      characterId: selectedValue
                  })}
                  anchorEl={this.menu}
                  ref={(el)=&gt;this.menu=el}
                  &gt;
                  Choose Character
              &lt;/SelectMenu&gt;
  </code></pre>