title: error : "Uncaught TypeError: Cannot read property 'length' of null" Chart.js
body: <p>I made several graphic with Chart.js that works. But the polar graphic and the radar does not work. I get the following error : "Uncaught TypeError: Cannot read property 'length' of null"</p>
  
  <p>I use charjs 2.3.0</p>
  
  <p>I do not understand where the error can come .</p>
  
  <pre><code>&lt;div class="col-lg-6 col-md-6"&gt;
       &lt;canvas class="box box-warning" id="myChart4" width="400" height="400"&gt;&lt;/canvas&gt;
  &lt;/div&gt;
  
  &lt;div class="col-lg-6 col-md-6"&gt;
       &lt;canvas class="box box-warning" id="myChart5" width="400" height="400"&gt;&lt;/canvas&gt;
  &lt;/div&gt;
  
  //////////////////////////////////////////////
  // Chart Polar
  //////////////////////////////////////////////
  
  var ctx = document.getElementById("myChart4");
  
  new Chart(ctx, {
      type: 'polarArea',
      data: {
          labels: [
              "Red",
              "Blue",
              "Yellow"
          ],
          datasets: [{
              data: [300, 50, 100],
              backgroundColor: [
                  "#FF6384",
                  "#36A2EB",
                  "#FFCE56"
              ],
              hoverBackgroundColor: [
                  "#FF6384",
                  "#36A2EB",
                  "#FFCE56"
              ]
          }]
      },
      options: {
          cutoutPercentage: 50,
          animation: {
              animateScale: false
          }
      }
  });
  
  //////////////////////////////////////////////
  // Radar
  //////////////////////////////////////////////
  
  var ctx = document.getElementById("myChart5");
  
  
  var scatterChart = new Chart(ctx, {
      type: 'radar',
      data: data = {
  
          labels: ["Lundi", "Mardi", "Mercredi", "Jeudi", "Vendredi", "Samedi", "Dimanche", ],
          datasets: [{
              label: 'the first dataset',
              backgroundColor: 'rgba(255, 99, 132, 0.2)',
              borderColor: 'rgba(255,99,132,1)',
              data: [10, 34, 50, 34, 56, 65, 43]
          }, {
              label: 'the second dataset',
  
              backgroundColor: 'rgba(54, 162, 235, 0.2)',
              borderColor: 'rgba(54, 162, 235, 1)',
  
              data: [54, 72, 100, 36, 76, 23, 21]
          }]
      },
      options: {
          scales: {
              xAxes: [{
                  type: 'linear',
                  position: 'bottom'
              }]
          }
      }
  });
  </code></pre>