java.lang.NullPointer尝试将admob放入应用程序时出现异常错误

Question

当我运行此代码尝试运行Admob广告时，我收到错误引起：billpliter.com.simplebillspliter.MainActivity.onCreate（MainActivity.java:30）中的java.lang.NullPointerException

(28)AdView mAdView = (AdView) findViewById(R.id.adView);
(29)AdRequest adRequest = new AdRequest.Builder().build();
(30)mAdView.loadAd(adRequest);

我正在尝试开发一个Android工作室应用程序

这是我的XML

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity"

xmlns:ads="http://schemas.android.com/apk/res-auto"


android:id="@+id/old">

<EditText
    android:hint="Enter The Cost Here"
    android:inputType="numberDecimal" android:id="@+id/num1"
    android:textSize="30sp"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:paddingBottom="40sp"
 />

<ImageView
    android:layout_width="match_parent"
    android:layout_height="80sp"
    android:src="@drawable/money"/>

<EditText
    android:hint="Enter The Number Of People Here"
    android:gravity="center"
    android:textSize="22sp"
    android:inputType="number"
    android:id="@+id/num2"
    android:layout_width="fill_parent"
    android:layout_height="90sp"

    android:nestedScrollingEnabled="true" />
<ImageView
    android:layout_width="match_parent"
    android:layout_height="80sp"
    android:src="@drawable/howmany"/>



<Button
    android:layout_width="match_parent"
    android:layout_height="100sp"
    android:text="How Much Do I Pay"
    android:id="@+id/button"
    android:layout_gravity="center_horizontal"
    android:layout_marginBottom="30sp"
    android:clickable="true"
    android:textSize="30sp" />


<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal">

    <TextView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:text="£"
    android:gravity="start"

    android:id="@+id/poundsign"

    android:textSize="50sp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text=""
        android:id="@+id/sum"

        android:gravity="end"
        android:textSize="50sp"/>

    </LinearLayout>

<com.google.android.gms.ads.AdView
    android:id="@+id/adView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    ads:adSize="BANNER"
    ads:adUnitId="@string/banner_ad_unit_id">
</com.google.android.gms.ads.AdView>
</LinearLayout>