要素削除してもgetComputedStyle()のエラーを出ないようにするためには？

下記のJavaScriptのコードの、pdiv.removeChild(numButton);(2箇所存在します)でpdivからnumButton要素を削除すると、以下のようなエラーがでます。

Uncaught TypeError: Failed to execute 'getComputedStyle' on 'Window': parameter 1 is not of type 'Element'.

var nBStyle = document.defaultView.getComputedStyle(nB);が原因ですが、どうすればいいのでしょうか？

JavaScript
1var PassSec;
2
3    function chgImg(pdiv){
4        //canvasの描画
5        var canvas = document.getElementById("screen");
6        var context = canvas.getContext("2d");
7        canvas.width = 460;
8        canvas.height = 345;
9        context.fillStyle = "#000000";
10        context.rect(0,0,canvas.width,canvas.height);
11        context.fill();
12
13        //canvasからimgに変換
14        var jpg = canvas.toDataURL();
15        var newImg = document.createElement("img");
16        newImg.src = jpg;
17        pdiv.replaceChild(newImg, canvas);
18    }
19
20    function Start(pdiv){
21        //タイトル
22        var newText = document.createElement("h1");
23        var newTContent = document.createTextNode("Simple Game");
24        newText.appendChild(newTContent);
25        newText.id = "title";
26        pdiv.appendChild(newText);
27
28        //スタートボタン
29        var newSButton = document.createElement("button");
30        var newSContent = document.createTextNode("START");
31        newSButton.appendChild(newSContent);
32        newSButton.id = "start";
33        pdiv.appendChild(newSButton);
34    }
35
36    function showPassage(){
37        PassSec++;
38
39        var sec = PassSec % 60;
40        var min = parseInt(PassSec / 60);
41
42        if(sec <= 9){
43            sec = "0" + sec;
44        }
45
46        var count = min + "：" + sec;
47        document.getElementById("watStyle").innerHTML = count;
48    }
49
50    function Move(){
51        do{
52            //位置決め
53            function getRandom(){
54                return Math.random();
55            }
56            var x = getRandom();
57            var y = getRandom();
58        }while((x * 429 > 352 && y * 324 > 287) || (x * 429 > 366 && y * 324 < 30));  //終了ボタンやタイマーと重なったらもう一度
59        var nB = document.getElementById("numbut");
60        var nBStyle = document.defaultView.getComputedStyle(nB);
61        nB.style.left = nBStyle.left;
62        nB.style.top = nBStyle.top;
63        nB.style.left = x * 429 + "px";
64        nB.style.top = y * 324 + "px";
65    }
66
67    window.onload = function(){
68        var pdiv = document.getElementById("parent-div");
69        chgImg(pdiv);
70        Start(pdiv);
71
72        document.getElementById("start").addEventListener("click", function(){
73            //前画面の要素削除
74            pdiv.removeChild(title);
75            pdiv.removeChild(start);
76
77            //時間経過
78            var watch = document.createElement("p");
79            watch.id = "watStyle";
80            pdiv.appendChild(watch);
81            watch.innerHTML = "0：00";
82            PassSec = 0;
83            PassSecID = setInterval("showPassage()", 1000);
84            
85            //数字ボタンの変遷（ゲーム内容）
86            var numButton = document.createElement("input");
87            numButton.type = "button";
88            numButton.value = "01";
89            num = numButton.value;
90            numButton.id = "numbut";
91            pdiv.appendChild(numButton);
92            Move();
93            timer = setInterval("Move()", 1000);
94            numButton.onclick = function(){
95                clearInterval(timer);
96                if(num < 9){
97                    numButton.value = "0" + ++num;
98                }else{
99                    numButton.value = ++num;
100                }
101                Move();
102                timer = setInterval("Move()", 1000);
103                if(numButton.value > 20){
104                    pdiv.removeChild(numButton);
105                    pdiv.removeChild(newEButton);
106                    
107                }
108            }
109
110            //終了ボタン
111            var newEButton = document.createElement("button");
112            var newEContent = document.createTextNode("Give up");
113            newEButton.appendChild(newEContent);
114            newEButton.id = "give-up";
115            pdiv.appendChild(newEButton);
116            newEButton.onclick = function(){
117                pdiv.removeChild(numButton);
118                pdiv.removeChild(newEButton);
119                clearInterval(PassSecID);
120                pdiv.removeChild(watch);
121            }
122        });
123    }

CSS
1#parent-div{
2    position: relative;
3    width: 460px;
4    height: 345px;
5}
6
7#title{
8    color: #00bfff;
9    font-family: sans-serif;
10    font-size: 45px;
11    text-align: center;
12    position: absolute;
13    top: 70px;
14    left: 0;
15    right: 0;
16    margin: auto;
17}
18
19#start{
20    position: absolute;
21    top: 230px;
22    left: 0;
23    right: 0;
24    margin: auto;
25}
26
27#give-up{
28    position: absolute;
29    top: 315px;
30    left: 390px;
31}
32
33#numbut{
34    position: absolute;
35    left: 0;
36    top: 0;
37}
38
39#watStyle{
40    position: absolute;
41    left: 405px;
42    top: 5px;
43    color: yellow;
44    margin: 0;
45}

HTML
1<!DOCTYPE html>
2<html>
3    <head>
4        <meta charset="utf-8">
5        <link rel="stylesheet" href="style.css">
6    </head>
7    <body>
8        <div id="parent-div">
9            <canvas id="screen"></canvas>
10        </div>
11        <script src="main.js"></script>
12    </body>
13</html>