android – 添加/删除RelativeLayout视图时的NullPointerException(动画)

嗨,我想从下到上创建一个图像和动画,并将alpha从1.0更改为0.0.我有这个代码(我在onCreate方法中调用3次来制作3个动画图像)：

/**
 * Starts to make fog
 */
private void startFogGenerator() {

    handler.postDelayed(new Runnable() {
        @Override
        public void run() {

            final ImageView img = new ImageView(Activity.this);

            Boolean showN1 = r.nextBoolean();
            if (showN1) {
                img.setImageresource(R.drawable.nube_01);
            } else {
                img.setImageresource(R.drawable.nube_02);
            }

            Animation animation = AnimationUtils.loadAnimation(Activity.this,R.anim.translate_and_alpha);
            animation.setAnimationListener(new Animation.AnimationListener() {
                @Override
                public void onAnimationStart(Animation animation) {
                }

                @Override
                public void onAnimationEnd(Animation animation) {

                    fogLayout.removeView(img);
                    fogLayout.invalidate();
                    // always true? Maybe but just check for for concurrency safe.
                    if (setFogN(false) < MAX_CLOUDS_ON_SCREEN) {
                        startFogGenerator();
                    }

                }

                @Override
                public void onAnimationRepeat(Animation animation) {
                }
            });


            int pxWidth = Tools.toPx(140,getresources()); // Width of layout container
            int paddingWidth = Tools.toPx(20,getresources());
            int x = r.nexTint(pxWidth - paddingWidth - paddingWidth) + paddingWidth;

            int range = Tools.toPx(50,getresources());

            int width;
            int height;
            if (showN1) { // nube01: 167x226
                width = r.nexTint(Tools.toPx(167,getresources()) - rangE) + range;
                height= r.nexTint(Tools.toPx(226,getresources()) - rangE) + range;
            } else { // nube01: 144x177
                width = r.nexTint(Tools.toPx(144,getresources()) - rangE) + range;
                height= r.nexTint(Tools.toPx(177,getresources()) - rangE) + range;
            }

            Log.d("Animation","X: " + x + ",Width: " + width + ",Height: " + height);

            //img.setPadding(x,0);
            RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(width,height);
            params.leftMargin = x;
            fogLayout.addView(img,params);
            img.setBACkgroundColor(Color.argb(255,r.nexTint(255),r.nexTint(255))); // just for debug
            img.startAnimation(animation);
            setFogN(true);

        }
    },r.nexTint(2500 - 200) + 200);

}
private final static int MAX_CLOUDS_ON_SCREEN = 3;
private AtomicInteger currentFogs = new AtomicInteger(0);
private synchronized int setFogN(Boolean increment) {

    int toReturn = increment ? currentFogs.incrementAndGet() : currentFogs.decrementAndGet();

    Log.d("TeaAnim","ToReturn: "+ toReturn);

    return toReturn;

}

工作正常但是当删除第3个视图并开始另一个时间时,它会崩溃并出现NullPointerException,可能是并发问题？我怎么解决它？

例外：

ERROR/AndroidRuntime(10736): FATAL EXCEPTION: main
    java.lang.NullPointerException
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2796)
    at android.view.View.getDisplayList(View.java:12648)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2910)
    at android.view.View.getDisplayList(View.java:12588)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2910)
    at android.view.View.getDisplayList(View.java:12588)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2910)
    at android.view.View.getDisplayList(View.java:12588)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2910)
    at android.view.View.getDisplayList(View.java:12588)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2910)
    at android.view.View.getDisplayList(View.java:12588)
    at android.view.View.getDisplayList(View.java:12694)
    at android.view.HardwareRenderer$GlRenderer.draw(HardwareRenderer.java:1198)
    at android.view.ViewRootImpl.draw(ViewRootImpl.java:2173)
    at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2045)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1854)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
    at android.view.Choreographer$CallBACkRecord.run(Choreographer.java:749)
    at android.view.Choreographer.doCallBACks(Choreographer.java:562)
    at android.view.Choreographer.doFrame(Choreographer.java:532)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
    at android.os.Handler.handleCallBACk(Handler.java:725)
    at android.os.Handler.dispatchmessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5204)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:799)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:566)
    at dalvik.system.NativeStart.main(Native Method)

笔记：
fogLayout是一个具有固定宽度和高度的RelativeLayout(以dp为单位)
Tools.toPx是一种从DP转换为PX的方法

handler.post(new Runnable() {
    public void run() {
        fogLayout.removeView(img);
    }
});