----------------------------------------
[20.06.28]更新
成功了謝謝各位大神的指導，小弟感激不盡。這次問題的重點是關於fragment的技巧，若是先了解就不會有這種問題。
----------------------------------------

照著這個影片做完後https://www.youtube.com/watch?v=BrDX6VTgTkg
出現以下錯誤，應用閃退

2020-06-26 16:03:49.125 2559-2559/? E/om.example.fsr: Unknown bits set in runtime_flags: 0x8000
2020-06-26 16:03:53.292 2559-2559/com.example.fsrc E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.fsrc, PID: 2559
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fsrc/com.example.fsrc.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
        at com.example.fsrc.MainActivity.onCreate(MainActivity.java:170)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

將以下程式註解後，繼續根據logcat註解出現錯誤的程式

recyclerView = findViewById(R.id.recyclerView);

        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        //recyclerView.setLayoutManager(layoutManager);
        //recyclerView.setHasFixedSize(true);

最後註解以下程式後恢復正常，但等於沒使用此功能，即無錯誤

recyclerAdapter = new RecyclerAdapter(getApplicationContext(), messagesList);
                //recyclerView.setAdapter(recyclerAdapter);
                recyclerAdapter.notifyDataSetChanged();

小弟求(詳)解~~~