队列错误“Uncaught TypeError：无法读取null的属性'对象'”

Question

我正在尝试使用队列来确保在脚本继续之前完全加载我的文件。但是，当我加载我的json映射文件并尝试在函数中使用它们时，我收到错误。

var counties = svg.append("g")
    .attr("id", "counties")
    .attr("class", "Blues");

var states = svg.append("g")
    .attr("id", "states");

    queue()
    .defer(d3.json, "county.json")
    .defer(d3.json, "state.json")
    .defer(d3.json, "Crashes.json")
    .await(ready);

function ready(county, state, crashes) {
    var countyFeatures = topojson.feature(county, county.objects.counties);
    var stateFeatures = topojson.feature(state, state.objects.states);

    /*
    counties.selectAll("path")
        .data(county.features)
      .enter().append("path")
        .attr("class", "q8-9")
        .attr("d", path);

    states.selectAll("path")
        .data(state.features)
      .enter().append("path")
        .attr("d", path);   
      */
}

错误为Uncaught TypeError: Cannot read property 'objects' of null

有人可以解释为什么在传入函数时它为空？而不是分别运行d3.json并使用

d3.json("URL", function(d) {var temp = topojson.feature(d, d.objects.counties;});

感谢。