安卓java.lang.IllegalArgumentException: The observer is null.解决方案

刚刚在调试自己的APP项目的时候报错java.lang.IllegalArgumentException: The observer is null.，而之前是可以运行通过，所以百思不得其解，后来在网上查找相关资料，终于弄明白是咋回事，现在记录如下，一来算是积累调错方面的知识，二来或许可以帮到出现同样错误的人。

出错的主要原因是在ViewPager中使用到了GridView这样的控件，当销毁包含该ViewPager控件的Activity时，如切换Fragment或Activity或退出应用，则会报java.lang.IllegalArgumentException: The observer is null.的错误，主要原因是GridView使用的是适配器模式来显示视图，采用观察者模式来更新视图，在移除View时会调用BaseAdapter.unregisterDataSetObserver方法，所以如果GridView在移除View时如果对同一个View移除多次，则会报java.lang.IllegalArgumentException:
The observer is null.错误。解决方案很简单，只需在GridView的适配器类中重写unregisterDataSetObserver方法，在其中加一个判空语句即可，注意该错误同样适用于ListView，因为ListView同样使用的是适配器模式。

错误代码如下：

java.lang.IllegalArgumentException: The observer is null.
	at android.database.Observable.unregisterObserver(Observable.java:64)
	at android.widget.BaseAdapter.unregisterDataSetObserver(BaseAdapter.java:42)
	at android.widget.AbsListView.onDetachedFromWindow(AbsListView.java:2872)
	at android.view.View.dispatchDetachedFromWindow(View.java:11908)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2647)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewGroup.dispatchDetachedFromWindow(ViewGroup.java:2645)
	at android.view.ViewRootImpl.dispatchDetachedFromWindow(ViewRootImpl.java:2685)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:4044)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:4027)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:375)
	at android.view.WindowManagerImpl$CompatModeWrapper.removeViewImmediate(WindowManagerImpl.java:170)
	at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3352)
	at android.app.ActivityThread.access$1200(ActivityThread.java:140)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1266)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loop(Looper.java:137)
	at android.app.ActivityThread.main(ActivityThread.java:4914)
	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:808)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:575)
	at dalvik.system.NativeStart.main(Native Method)

解决代码如下：

public class GridViewAdapter extends SimpleAdapter {

	public GridViewAdapter(Context context, List<? extends Map<String, ?>> data, int resource, String[] from,
			int[] to) {
		super(context, data, resource, from, to);
		// TODO Auto-generated constructor stub
	}
	@Override
	public void unregisterDataSetObserver(DataSetObserver observer) {
		if (observer != null) {
	        super.unregisterDataSetObserver(observer);
	    }
	}
}

即在自己适配器类中重写unregisterDataSetObserver方法，在其中加一个判空语句，如果observer不为空，才调用super.unregisterDataSetObserver(observer);语句。