使用window.onload = function(){}仍然无法解决“按钮” - 编程论坛

程序代码：

<!DOCTYPE HTML >
<HTML>

<HEAD>
  <TITLE> 实验一下 </TITLE>

</HEAD>

<BODY>
        <H2>员工信息列表</H2>
        <input type = "button" value = "显示员工信息列表" id = "displayBtn "/>
        <hr>
        <table border = 1px width = "50%">
            <tr>
                <th>员工编号</th>
                <th>员工名字</th>
                <th>员工薪资</th>
            </tr>
            <TBODY id = "emptbody">
                
            </TBODY>
        </table>
        总共<span id = "count">0</span>条数

                <script type = "text/javascript">
            //有这些json数据
            var data = {
                "total" :4,
                "emps" : [
                {"empno" : 7369, "ename" : "SMITH" , "sal":800.0},
                {"empno" : 7361, "ename" : "SMITH2" , "sal":1800.0},
                {"empno" : 7362, "ename" : "SMITH3" , "sal":2808.0},
                {"empno" : 7363, "ename" : "SMITH4" , "sal":3800.0}
                ]
            }


            //希望把数据展示到table中
            window.onload = function(){
                    //alert(11);
                document.getElementById("displayBtn").onclick = function(){
                    var emps = data.emps;
                    var html = "";
                    for(var i = 0; i <emps.length; i++){
                        var emp = emps[i];
                        html += "<tr>";
                        html += "<td>"+emp.empno+"</td>";
                        html += "<td>"+emp.ename+"</td>";
                        html += "<td>"+emp.sal+"</td>";
                        html += "</tr>";
                    }
                    document.getElementById("emptbody").innerHTML = html;
                    document.getElementById("count").innerHTML = data.total;

                }

            }
        </script>

</BODY>
</HTML>

按下按钮“显示员工信息列表”却不能将详细信息显示出来，控制台显示“Uncaught TypeError: Cannot set property 'onclick' of null
at window.onload”，我有使用window.onload啊，也有将onclick放到下面来。