使用BaseAdapter随机化形状颜色

Question

public View getView(int position, View convertView, ViewGroup parent) {
        View row;
        row = inflater.inflate(R.layout.item_view, parent, false);

        round = row.getResources().getDrawable(R.drawable.round);
        Random rnd = new Random();
        int color = Color.argb(255, rnd.nextInt(256), rnd.nextInt(256), rnd.nextInt(256));
        LayerDrawable bgDrawable = (LayerDrawable)row.findViewById(R.id.imgColor).getBackground();
        final GradientDrawable shape = (GradientDrawable) bgDrawable.findDrawableByLayerId(R.id.round_id);
        shape.setColor(color);
        return row;
    }

这是扩展BaseAdapter的类的一部分， 我正在尝试生成随机颜色并将其放在一个圆圈内，这是放在listView中，这就是我创建适配器的原因。错误可在以下代码行中找到：

final GradientDrawable shape = (GradientDrawable) bgDrawable.findDrawableByLayerId(R.id.round_id);

logcat的：

11-24 08:09:51.997    2438-2438/com.example.den.assignment02 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.den.assignment02, PID: 2438
    java.lang.NullPointerException
            at com.example.den.assignment02.CustomAdapter.getView(CustomAdapter.java:62)
            at android.widget.AbsListView.obtainView(AbsListView.java:2263)
            at android.widget.ListView.measureHeightOfChildren(ListView.java:1263)
            at android.widget.ListView.onMeasure(ListView.java:1175)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1916)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1295)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
            at android.view.Choreographer.doCallbacks(Choreographer.java:574)
            at android.view.Choreographer.doFrame(Choreographer.java:544)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

形状

<?xml version="1.0" encoding="utf-8"?>
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval"
    android:id="@+id/round_id">

    <solid
        android:color="#666666"/>

    <size
        android:width="60dp"
        android:height="60dp"/>
</shape>