Attempt to invoke virtual method on a null を解決したい

タイトルの通り、Attempt to invoke virtual method on a null を解決したいです。

ソースとログ

以下Logです:

03-22 16:09:13.525 17928-17928/com.kitunebi.sunsolararrow.flashanzan E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                       Process: com.kitunebi.sunsolararrow.flashanzan, PID: 17928
                                                                                       java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.kitunebi.sunsolararrow.flashanzan/com.kitunebi.sunsolararrow.flashanzan.MathActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
                                                                                           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2345)
                                                                                           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)
                                                                                           at android.app.ActivityThread.access$900(ActivityThread.java:153)
                                                                                           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)
                                                                                           at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                           at android.os.Looper.loop(Looper.java:148)
                                                                                           at android.app.ActivityThread.main(ActivityThread.java:5451)
                                                                                           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 'android.view.View android.view.Window.findViewById(int)' on a null object reference
                                                                                           at android.app.Activity.findViewById(Activity.java:2139)
                                                                                           at com.kitunebi.sunsolararrow.flashanzan.MathActivity.<init>(MathActivity.java:19)
                                                                                           at java.lang.Class.newInstance(Native Method)
                                                                                           at android.app.Instrumentation.newActivity(Instrumentation.java:1068)
                                                                                           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2335)
                                                                                           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494) 
                                                                                           at android.app.ActivityThread.access$900(ActivityThread.java:153) 
                                                                                           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347) 
                                                                                           at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                           at android.os.Looper.loop(Looper.java:148) 
                                                                                           at android.app.ActivityThread.main(ActivityThread.java:5451) 
                                                                                           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) 

関係ありそうなソースコードです。
MathActivity.java

//パッケージを定義したりインポートするコードは省略

/** This activity sets a question. */
public class MathActivity extends Activity {
    /** timer. */
    Timer timer;
    /** It displays a number. */
    TextView tv;       /19行目
    /**The number of times change the text.*/
    int kutisu;
    /**Max number.*/
    int max;
    /**The speed change the text*/
    int speed;
    /**The answer*/
    int answer;
    /**Handler. This is used in order to change the text(UI). */
    Handler mHandler = new Handler();
    /**onCreate method. */
    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_plusminus);
        this.tv=(TextView)findViewById(R.id.set);
    }
    protected void onResume(){
        super.onResume();

        //This code initializes kutisu, max, and speed.
        Intent intent = getIntent();
        this.kutisu = Integer.parseInt(intent.getStringExtra("kutisu"));
        this.max = Integer.parseInt(intent.getStringExtra("max"));
        speed(Integer.parseInt(intent.getStringExtra("speed")));

        startTimer();//Start a timer

        //Start a new activity
        intent.setClassName(this,"AnswerActivity");
        intent.putExtra("answer", answer);
        startActivity(intent);
    }
    //speedメソッドやstartTimerメソッド。省略
  ・
  ・
  ・
}

レイアウト

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="願いましては"
        android:id="@+id/set"
       <!--定義しているのですが--->
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:textSize="50dp" />
</RelativeLayout>

android:id="@+id/set"と書いたのですが、エラーが出ています。また、[これ](https//teratail.com/questions/34048)を見ましたが原因が違いそうです。(もしも原因が一緒ならすみません)