NullPointerException android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState

今天在使用android.support.v4.view.ViewPager和Fragment时，发现程序运行期间总是意外退出，报异常

07-20 15:44:59.453  31957-31957/com.zhang.ChatTab W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x40aa6300)

07-20 15:44:59.453  31957-31957/com.zhang.ChatTab E/AndroidRuntime﹕ FATAL EXCEPTION: main

    java.lang.NullPointerException

            at android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState(FragmentManager.java:1576)

            at android.support.v4.app.FragmentManagerImpl.saveAllState(FragmentManager.java:1617)

            at android.support.v4.app.FragmentActivity.onSaveInstanceState(FragmentActivity.java:481)

            at android.app.Activity.performSaveInstanceState(Activity.java:1137)

            at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1215)

            at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3079)

            at android.app.ActivityThread.handleStopActivity(ActivityThread.java:3138)

            at android.app.ActivityThread.access$900(ActivityThread.java:144)

            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1232)

            at android.os.Handler.dispatchMessage(Handler.java:99)

            at android.os.Looper.loop(Looper.java:137)

            at android.app.ActivityThread.main(ActivityThread.java:4966)

            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:791)

            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)

            at dalvik.system.NativeStart.main(Native Method)

百思不得其解，最终查阅相关资料，找到相关两种的解决方法：

一.在每个Fragment的布局文件中每个View必须分配一个ID，包括主布局控件。

二.在继承Fragment的类中，重载onSaveInstanceState方法，并且向outState中传入任意内容，如outState.putString("test","test")

至于其中的原理就不太明白了，哪位大牛知道了，也希望可以给予解释下