Unable to show Interstitial ad

Question

I've gone through all the previous answers on this question , but my error and approach is slightly different from the previous questions'.

I've only three views in my app : Login , Signup and Main. I want to show an interstitial ad every time I transition from one view to another.

In order to accomplish this, I'm initializing , loading and showing the ad at the start of each view:

MainActivity.java

package com.brain.passive;

import android.content.Intent;
import android.util.Log;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.ads.*;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.android.gms.ads.initialization.InitializationStatus;
import com.google.android.gms.ads.initialization.OnInitializationCompleteListener;
import com.google.android.gms.ads.interstitial.InterstitialAd;
import com.google.android.gms.ads.interstitial.InterstitialAdLoadCallback;

public class MainActivity extends AppCompatActivity {
    public EditText emailId, passwd;
    Button btnSignUp;
    TextView signIn;
    FirebaseAuth firebaseAuth;
    private FirebaseAuth.AuthStateListener authStateListener;
    private InterstitialAd mInterstitialAd;

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

        MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
            }
        });


        AdRequest adRequest = new AdRequest.Builder().build();

        InterstitialAd.load(this,"ca-app-pub-7147930999992278/2850010809", adRequest,
                new InterstitialAdLoadCallback() {
                    @Override
                    public void onAdLoaded(@NonNull InterstitialAd interstitialAd) {
                        // The mInterstitialAd reference will be null until
                        // an ad is loaded.
                        mInterstitialAd = interstitialAd;
                        Log.i("Loaded", "onAdLoaded");
                    }

                    @Override
                    public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
                        // Handle the error
                        Log.i("FailedError", loadAdError.getMessage());
                        mInterstitialAd = null;
                    }
                });

        mInterstitialAd.setFullScreenContentCallback(new FullScreenContentCallback(){
            @Override
            public void onAdDismissedFullScreenContent() {
                // Called when fullscreen content is dismissed.
                Log.d("TAG", "The ad was dismissed.");
            }

            @Override
            public void onAdFailedToShowFullScreenContent(AdError adError) {
                // Called when fullscreen content failed to show.
                Log.d("TAG", "The ad failed to show.");
            }

            @Override
            public void onAdShowedFullScreenContent() {
                // Called when fullscreen content is shown.
                // Make sure to set your reference to null so you don't
                // show it a second time.
                mInterstitialAd = null;
                Log.d("TAG", "The ad was shown.");
            }
        });

        if (mInterstitialAd != null) {
            mInterstitialAd.show(MainActivity.this);
        } else {
            Log.d("TAG", "The interstitial ad wasn't ready yet.");
        }
  
    \\Rest of the logic for App.

    }
}

LoginActivity.java and SignupActivity.java follow the same format (Initializing and displaying the ad first then the rest of the logic).

The problem is that the ads are not being displayed. Here are the error logs:

E/cr_VariationsUtils: Failed reading seed file "/data/user/0/com.brain.passive/app_webview/variations_seed": /data/user/0/com.brain.passive/app_webview/variations_seed (No such file or directory)

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.brain.passive/com.brain.passive.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void c.b.b.a.a.x.a.b(c.b.b.a.a.k)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void c.b.b.a.a.x.a.b(c.b.b.a.a.k)' on a null object reference
        at com.brain.passive.MainActivity.onCreate(:2)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

In verbose these lines are also shown:

W/GooglePlayServicesUtil: Google Play services out of date for com.brainiac.passiveincome.  Requires 212800000 but found 202414022

W/GoogleApiManager: The service for c.b.b.a.h.c.c is not available: b{statusCode=SERVICE_VERSION_UPDATE_REQUIRED, resolution=null, message=null}


W/VideoCapabilities: Unrecognized profile 4 for video/hevc
I/VideoCapabilities: Unsupported profile 4 for video/mp4v-es
W/cr_MediaCodecUtil: HW encoder for video/avc is not available on this device.

I'm unable to figure out if this is a video incompatibility problem or if I've made some mistake in the google ads setup. Pls help.