竖屏切换横屏时报异常：java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference

[JaquanGit]

进入PhotoPickerActivity，由竖屏切换到横屏时出现了异常，以下是异常信息：

java.lang.RuntimeException: Unable to start activity ComponentInfo{org.zuicool.studentapp/me.iwf.photopicker.PhotoPickerActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2448)
                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510)
                                     at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4226)
                                     at android.app.ActivityThread.access$1300(ActivityThread.java:163)
                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1381)
                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                     at android.os.Looper.loop(Looper.java:135)
                                     at android.app.ActivityThread.main(ActivityThread.java:5595)
                                     at java.lang.reflect.Method.invoke(Native Method)
                                     at java.lang.reflect.Method.invoke(Method.java:372)
                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964)
                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
                                     at me.iwf.photopicker.fragment.PhotoPickerFragment.onDetach(PhotoPickerFragment.java:235)
                                     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1202)
                                     at android.support.v4.app.FragmentManagerImpl.removeFragment(FragmentManager.java:1349)
                                     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:699)
                                     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617)
                                     at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:570)
                                     at me.iwf.photopicker.PhotoPickerActivity.onCreate(PhotoPickerActivity.java:75)
                                     at android.app.Activity.performCreate(Activity.java:6100)
                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1109)
                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401)
                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510) 
                                     at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4226) 
                                     at android.app.ActivityThread.access$1300(ActivityThread.java:163) 
                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1381) 
                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                     at android.os.Looper.loop(Looper.java:135) 
                                     at android.app.ActivityThread.main(ActivityThread.java:5595) 
                                     at java.lang.reflect.Method.invoke(Native Method) 
                                     at java.lang.reflect.Method.invoke(Method.java:372) 
                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964) 
                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759) 

请问这是什么原因？