애드몹 전면광고 실행시 오류 질문입니다

Question

package com.sendiagodis.administrator.myapplication;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import com.google.android.gms.ads.AdListener;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import com.google.android.gms.ads.InterstitialAd;
import com.google.android.gms.ads.MobileAds;
public class MainActivity extends AppCompatActivity {
    private AdView mAdView;
    private InterstitialAd mInterstitialAd;
    private InterstitialAd mInterstitial;
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button testbutton = (Button)findViewById(R.id.button1push);
        testbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (mInterstitial.isLoaded()){
                    mInterstitial.show();
                }
                else {
                    Log.d("TAG", "The interstitial wasn't loaded yet.");}
            }
        });




        mAdView = findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().addTestDevice("76E6661E84EDA290FA26D9A88FB46DAD").build();
        mAdView.loadAd(adRequest);

        MobileAds.initialize(this, "ca-app-pub-3940256099942544~3347511713");


        mInterstitialAd = new InterstitialAd(this);
        mInterstitialAd.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
        mInterstitialAd.loadAd(new AdRequest.Builder().build());

        mInterstitialAd.setAdListener(new AdListener() {
            @Override
            public void onAdClosed() {
                // Load the next interstitial.
                mInterstitialAd.loadAd(new AdRequest.Builder().build());
            }

        });

        mInterstitialAd.setAdListener(new AdListener() {
            @Override
            public void onAdClosed() {
                // Load the next interstitial.
                mInterstitialAd.loadAd(new AdRequest.Builder().build());
            }

        });

    }
    public void button1push(View view)
    {
        Intent intent = new Intent(this, a_page.class);
        startActivity(intent);
    }

}

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.sandiagodis.man1.myapplication, PID: 25942
                  java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.google.android.gms.ads.InterstitialAd.isLoaded()' on a null object reference
                      at com.sendiagodis.administrator.myapplication.MainActivity$1.onClick(MainActivity.java:29)
                      at android.view.View.performClick(View.java:6205)
                      at android.widget.TextView.performClick(TextView.java:11103)
                      at android.view.View$PerformClick.run(View.java:23653)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6682)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)
Application terminated.

 

버튼 클릭시 이렇게 뜨는데 뭐가 문제인지 모르겠네요 ㅠㅠ