Не могу решить проблему в Android

Начал изучать android. Столкнулся с проблемой, когда добавляю интерфейс к MainActivity, Android Studio выдает ошибку.При этом Gradle собирается без ошибок. Сам код

 package com.example.musicshop;

 import android.os.Bundle;
 import android.view.View;
 import android.widget.AdapterView;
 import android.widget.ArrayAdapter;
 import android.widget.Spinner;
 import android.widget.TextView;

 import androidx.appcompat.app.AppCompatActivity;

 import java.util.ArrayList;
 import java.util.HashMap;


 public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener {
 int quantiti = 0;
 Spinner spinner;
 ArrayList spinnerArrayList;
 ArrayAdapter spinnerAdapter;
 HashMap goodsMap;
 String goodsName;
 double price;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    spinner = findViewById(R.id.spinnerItem);
    spinner.setOnItemSelectedListener(this);
    spinnerArrayList = new ArrayList();
    spinnerArrayList.add("guitar");
    spinnerArrayList.add("drumnbase");
    spinnerArrayList.add("keyboard");

    spinnerAdapter = new ArrayAdapter(this, android.R.layout.simple_spinner_item, spinnerArrayList);
    spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_item);
    spinner.setAdapter(spinnerAdapter);

    goodsMap = new HashMap();
    goodsMap.put("guitar", 500.0);
    goodsMap.put("drumnbase", 1500.0);
    goodsMap.put("keyboard", 1000.0);


}

public void increaseQuntity(View view) {
    quantiti = quantiti + 1;
    TextView textViewZero = findViewById(R.id.TextViewZero);
    textViewZero.setText(""   + quantiti);

}

public void uncreaseQuantiti(View view) {
    quantiti = quantiti - 1;
    if (quantiti < 0){
        quantiti = 0;
    }
    TextView textViewZero = findViewById(R.id.TextViewZero);
    textViewZero.setText(""   + quantiti);

}


@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
    goodsName = spinner.getSelectedItem().toString();
    price = (double)goodsMap.get(goodsName);
    TextView priceTextView = findViewById(R.id.textViewPrice);
    priceTextView.setText("" + quantiti * price);

}

@Override
public void onNothingSelected(AdapterView<?> parent) {

}

}

И ошибка

2020-03-10 20:13:01.496 15423-15423/com.example.musicshop E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.musicshop, PID: 15423
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.musicshop/com.example.musicshop.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2843)
    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 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
    at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:159)
    at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
    at android.content.Context.obtainStyledAttributes(Context.java:675)
    at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
    at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
    at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
    at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
    at com.example.musicshop.MainActivity.<init>(MainActivity.java:11)
    at java.lang.Class.newInstance(Native Method)
    at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
    at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:41)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1215)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
    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) 

Помогите разобраться плиз, уже несколько часов пытаюсь в интернете найти решение, но ничего не спасает.