Invoking on a Null Object Reference -- Android Studio Electric Eel

Question

I'm building an app for a homework assignment here. The app keeps failing to launch because of an error. I know this is for an error that some others have experienced, but I can't understand how to apply it to my own, which is why I'm asking it here.

Here is the error:

AndroidRuntime: FATAL EXCEPTION: main
Process: wesfritz.c196, PID: 4260
java.lang.RuntimeException: Unable to start activity ComponentInfo{wesfritz.c196/wesfritz.c196.UI.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at wesfritz.c196.UI.MainActivity.onCreate(MainActivity.java:21)
    at android.app.Activity.performCreate(Activity.java:8000)
    at android.app.Activity.performCreate(Activity.java:7984)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Here's the MainActivity.java code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //This is how buttons are set up in Android
    Button button=this.findViewById(R.id.enterButton);
    button.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            Intent intent = new Intent(MainActivity.this, TermList.class);
            //Intent is what connects functions together - makes a thing do a thing!
            startActivity(intent);
        }
    });
    setContentView(R.layout.activity_main);
}

Here is what the activity_main.xml looks like:

<TextView
    android:id="@+id/textView"
    android:layout_width="124dp"
    android:layout_height="56dp"
    android:fontFamily="sans-serif-condensed-medium"
    android:text="StudentApp"
    android:textAlignment="center"
    android:textAppearance="@style/TextAppearance.AppCompat.Large"
    android:textStyle="bold"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.498"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.578" />

<ImageView
    android:id="@+id/imageView"
    android:layout_width="156dp"
    android:layout_height="150dp"
    app:layout_constraintBottom_toTopOf="@+id/textView"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.537"
    app:srcCompat="@drawable/ic_launcher_foreground" />

<Button
    android:id="@+id/enterButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Enter"
    android:clickable="true"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/textView" />

I've tried looking at other examples on here, but they're different and I haven't been able to figure out why mine won't launch. I thought it was because I have a method in a method, but the teacher wrote that code portion. I see that the line that has the problem is the "setOnClickListener" line, but I don't understand. Also, I think "enterButton" is correct across my XML and Java pieces. I'm kind of at a loss.