Tabbed Activity スピナー実装方法

Question

TabbedActivityを用いて、タブで画面を作成し、その一つの項目にスピナーで都道府県を選択できるアプリを作成しています。
しかし、スピナーの実装でエラーが出て実行ができません。
タブの内部でスピナーを実装するのに変更するべき点、
用いるべき手法などがあればお聞きしたいと思います。
以下がソースコードになります。

MainActivity.java

import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Spinner;

public class MainActivity extends AppCompatActivity {

    String prefectures;

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    private SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);

        mViewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
        tabLayout.addOnTabSelectedListener(new TabLayout.ViewPagerOnTabSelectedListener(mViewPager));



    }


    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            switch(position) {
                case 0:
                    TabOne tabOne = new TabOne();
                    return tabOne;
                case 1:
                    TabTwo tabTwo = new TabTwo();
                    return tabTwo;
                default:
                    return null;
            }
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 2;
        }
    }
}

TabTwo.java

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;

/**
 * Created by h3a1 on 2018/01/22.
 */

public class TabTwo extends Fragment {

    String prefectures;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.tab_two,container,false);


        return view;

    }
}

tab_two.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical">

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="match_parent"
        android:layout_height="0dip"
        android:layout_weight="1"
        android:background="@android:drawable/list_selector_background"
        android:entries="@array/txt_area"
        android:spinnerMode="dialog"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        tools:layout_editor_absoluteX="8dp"
        tools:layout_editor_absoluteY="166dp" />

    <EditText
        android:id="@+id/cityEdit"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:ems="10"
        android:hint="市区町村を絞り込む場合は市区町村を入力"
        android:inputType="textPersonName" />

    <Button
        android:id="@+id/fucuoka"
        android:layout_width="fill_parent"
        android:layout_height="0dip"
        android:layout_weight="1"
        android:text="地域を選択して検索"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        tools:layout_editor_absoluteY="468dp" />
</LinearLayout>

エラー

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example..tabbedactivity, PID: 25508
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tabbedactivity/com.example.h3a1.tabbedactivity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                      at android.app.ActivityThread.-wrap11(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference
                      at com.example.h3a1.tabbedactivity.MainActivity.onCreate(MainActivity.java:58)
                      at android.app.Activity.performCreate(Activity.java:6251)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                      at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:5417) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Answer 1

 //スピナー実装
        Spinner spinner = (Spinner) findViewById(R.id.spinner);
        spinner.setOnItemSelectedListener
                (new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                Spinner spinner = (Spinner) parent;
                prefectures = (String)spinner.getSelectedItem();
            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) { }
        });

これをTabTwo.javaのonCreatView内に書き込み、
findViewByIdの前にviewを書くと解決しました。

Answer 2

MainActivity.javaの58行目でヌル参照例外が発生しています。
この例外はタブとかスピナー以前の問題で、コーディングミスによるものだと思います。
該当箇所で使用しているインスタンスを初期化してください。