App is getting crashed when it add fragment_container and fragment

Android Programming (3rd Edition) 8. Displaying Lists with RecyclerView
1

package com.developer.criminalintent;

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;

public abstract class SingleFragmentActivity extends AppCompatActivity {

protected abstract Fragment createFragment();


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_fragment);

    FragmentManager fm = getSupportFragmentManager();
    Fragment fragment = fm.findFragmentById(R.id.fragment_container);
    if (fragment == null)
    {
        fragment = createFragment();
        fm.beginTransaction()
                .add(R.id.fragment_container , fragment)
                .commit();

    }

}

}

and this is logcat errors:
05-10 20:32:16.409 8438-8438/com.developer.criminalintent E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.developer.criminalintent, PID: 8438
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.developer.criminalintent/com.developer.criminalintent.CrimeListActivity}: java.lang.IllegalStateException: Fragment com.developer.criminalintent.CrimeListFragment must be a public static class to be properly recreated from instance state.
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.IllegalStateException: Fragment com.developer.criminalintent.CrimeListFragment must be a public static class to be properly recreated from instance state.
at androidx.fragment.app.FragmentTransaction.doAddOp(FragmentTransaction.java:165)
at androidx.fragment.app.BackStackRecord.doAddOp(BackStackRecord.java:179)
at androidx.fragment.app.FragmentTransaction.add(FragmentTransaction.java:134)
at com.developer.criminalintent.SingleFragmentActivity.onCreate(SingleFragmentActivity.java:24)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Another classes are exactly same as book but it getting crashed when it want add R.id.fragment_container and fragment.
IF YOU CAN PLEASE HELP ME.