프래그먼트 질문있습니다.

Question

 

안드로이드 스튜디오로 프로젝트 생성할때 Bottom Navi뭐시기로 만들고 네비게이션 버튼이 눌러지면 프레그먼트가 전환되로록 할려고 해봤는데 그냥 꺼집니다. 질문할때 필요한 정보가 뭔지 몰라 일단 메인 엑티비티에서 작성한 코드만 올려봅니다. 아래는 로그켓에서 뜨던 에러메세지입니다.

Process: com.example.myapplication, PID: 23739
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to write to field 'android.app.FragmentManagerImpl android.app.Fragment.mFragmentManager' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2917)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2978)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1628)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6646)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
Caused by: java.lang.NullPointerException: Attempt to write to field 'android.app.FragmentManagerImpl android.app.Fragment.mFragmentManager' on a null object reference
at android.app.BackStackRecord.doAddOp(BackStackRecord.java:465)
at android.app.BackStackRecord.add(BackStackRecord.java:455)
at com.example.myapplication.MainActivity.onCreate(MainActivity.java:49)
at android.app.Activity.performCreate(Activity.java:6912)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2870)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2978) 
at android.app.ActivityThread.-wrap14(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1628) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6646) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)