Kotlin：viewPager must not be null

今天在寫 kotlin 的時候遇到“ XXX must not be null ”，長時間不能解決，報錯如下：

Process: com.ben.kotlindemo, PID: 12695
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ben.kotlindemo/com.ben.kotlindemo.MainActivity}: java.lang.IllegalStateException: viewPager must not be null
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2740)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2801)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1548)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:163)
        at android.app.ActivityThread.main(ActivityThread.java:6384)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:791)
     Caused by: java.lang.IllegalStateException: viewPager must not be null
        at com.ben.kotlindemo.fragment.CalendarFragment.initView(CalendarFragment.kt:88)
        at com.ben.kotlindemo.fragment.CalendarFragment.onCreateView(CalendarFragment.kt:67)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:2346)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1428)
        at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827)
        at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
        at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)
        at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)
        at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)
        at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3248)
        at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3200)
        at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:195)
        at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:597)
        at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1249)
        at android.app.Activity.performStart(Activity.java:6887)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2801) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1548) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:163) 
        at android.app.ActivityThread.main(ActivityThread.java:6384) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:791) 

解決辦法，使用 findViewById() ，如下就可以了

 val viewPager: ViewPager = view.findViewById(R.id.viewPager) 

具體原因還在查找中…