title: Reference error in d3.js code
body: <p>I am new to javascript as well as d3. I am running one of the graph codes from the website but I am getting the following error: </p>
  
  <p>Uncaught TypeError: Cannot read property 'children' of null d3.v3.min.js:2
  gu                                                          d3.v3.min.js:2
  n                                                           d3.v3.min.js:4
  e                                                           d3.v3.min.js:4
  e                                                           d3.v3.min.js:4
  (anonymous function)                                        index.html:44
  (anonymous function)                                        d3.v3.min.js:1
  t                                                           d3.v3.min.js:1
  u                                                           d3.v3.min.js:1</p>
  
  <p>Here's the whole code: </p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;meta charset="utf-8"&gt;
  &lt;style&gt;
  
  path {
    stroke: #fff;
    fill-rule: evenodd;
  }
  
  &lt;/style&gt;
  &lt;body&gt;
  &lt;script src="http://d3js.org/d3.v3.min.js"&gt;&lt;/script&gt;
  &lt;script&gt;
  
  var width = 960,
      height = 700,
      radius = Math.min(width, height) / 2;
  
  var x = d3.scale.linear()
      .range([0, 2 * Math.PI]);
  
  var y = d3.scale.sqrt()
      .range([0, radius]);
  
  var color = d3.scale.category20c();
  
  var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height)
    .append("g")
      .attr("transform", "translate(" + width / 2 + "," + (height / 2 + 10) + ")");
  
  var partition = d3.layout.partition()
      .value(function(d) { return d.size; });
  
  var arc = d3.svg.arc()
      .startAngle(function(d) { return Math.max(0, Math.min(2 * Math.PI, x(d.x))); })
      .endAngle(function(d) { return Math.max(0, Math.min(2 * Math.PI, x(d.x + d.dx))); })
      .innerRadius(function(d) { return Math.max(0, y(d.y)); })
      .outerRadius(function(d) { return Math.max(0, y(d.y + d.dy)); });
  
  d3.json("http://bl.ocks.org/d/4063550/flare.json", function(root) {
    var path = svg.selectAll("path")
        .data(partition.nodes(root))
      .enter().append("path")
        .attr("d", arc)
        .style("fill", function(d) { return color((d.children ? d : d.parent).name); })
        .on("click", click);
  
    function click(d) {
      path.transition()
        .duration(750)
        .attrTween("d", arcTween(d));
    }
  });
  
  d3.select(self.frameElement).style("height", height + "px");
  
  // Interpolate the scales!
  function arcTween(d) {
    var xd = d3.interpolate(x.domain(), [d.x, d.x + d.dx]),
        yd = d3.interpolate(y.domain(), [d.y, 1]),
        yr = d3.interpolate(y.range(), [d.y ? 20 : 0, radius]);
    return function(d, i) {
      return i
          ? function(t) { return arc(d); }
          : function(t) { x.domain(xd(t)); y.domain(yd(t)).range(yr(t)); return arc(d); };
    };
  }
  
  &lt;/script&gt;
  </code></pre>