title: Mapbox Draw (Custom Mode) - Why is changeMode() on draw undefined?
body: <p>I want to use a custom mode in Mapbox Draw. I declare the draw object like this:</p>
  
  <pre><code>  var draw = new MapboxDraw({
          displayControlsDefault: false,
          controls: {
              polygon: false,
              line_string: true,
              point: false,
              trash: true,
          },
          modes: Object.assign({
              custom_mode: CustomMode,
          }, MapboxDraw.modes),
      });
  </code></pre>
  
  <p>Then I want to change the mode:</p>
  
  <pre><code>map.on('load', function () {
      draw.changeMode("custom_mode");
      map.addControl(draw, 'bottom-right');
  });
  </code></pre>
  
  <p>Result: Error: Cannot read property 'changeMode' of undefined.
  But I can log the draw object just before the changeMode() call.
  What is the problem here? </p>