The application crashes when switching to activity

Hello, I am a novice programmer. And I decided to make a project in which examples will be created on the choice of complexity. I created a class and created a key and constants in it, and in activity, I switch to another activity, depending on the pressed button, create an example. But for some reason, when choosing the complexity of the application crashes. Logs:

FATAL EXCEPTION: main E AndroidRuntime Process: com.konter.random, PID: 14168 E AndroidRuntime java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.konter.random/com.konter.random.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Intent.getIntExtra(java.lang.String, int)' on a null object reference E AndroidRuntime at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269) E AndroidRuntime at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420) E AndroidRuntime at android.app.ActivityThread.access$900(ActivityThread.java:154) E AndroidRuntime at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321) E AndroidRuntime at android.os.Handler.dispatchMessage(Handler.java:102) E AndroidRuntime at android.os.Looper.loop(Looper.java:135) E AndroidRuntime at android.app.ActivityThread.main(ActivityThread.java:5294) E AndroidRuntime at java.lang.reflect.Method.invoke(Native Method) E AndroidRuntime at java.lang.reflect.Method.invoke(Method.java:372) E AndroidRuntime at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) E AndroidRuntime at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) E AndroidRuntime at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:102) E AndroidRuntime Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Intent.getIntExtra(java.lang.String, int)' on a null object reference E AndroidRuntime at com.konter.random.MainActivity.<init>(MainActivity.java:120) E AndroidRuntime at java.lang.reflect.Constructor.newInstance(Native Method) E AndroidRuntime at java.lang.Class.newInstance(Class.java:1606) E AndroidRuntime at android.app.Instrumentation.newActivity(Instrumentation.java:1066) E AndroidRuntime at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2259) E AndroidRuntime ... 11 more 

HomeActivity (first activation):

 public class HomeActivity extends Activity { Button play,easyBuutton,mediumButton,hardButton; LinearLayout lin; LinearLayout lin2; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.home); play=(Button)findViewById(R.id.homeButton); easyBuutton=(Button)findViewById(R.id.easyButton); mediumButton=(Button)findViewById(R.id.mediumButton); hardButton=(Button)findViewById(R.id.hardButton); lin=(LinearLayout)findViewById(R.id. outAnim); lin2=(LinearLayout)findViewById(R.id.inAnim); final Animation fallingAnimation = AnimationUtils.loadAnimation(this,R.anim.one); final Animation fallingAnimation1 = AnimationUtils.loadAnimation(this,R.anim.two); fallingAnimation.setAnimationListener(new Animation.AnimationListener(){ @Override public void onAnimationStart(Animation p1) { // TODO: Implement this method } @Override public void onAnimationEnd(Animation p1) { lin.setVisibility(View.GONE); lin2.setVisibility(View.VISIBLE); lin2.startAnimation(fallingAnimation1); } @Override public void onAnimationRepeat(Animation p1) { // TODO: Implement this method } }); play.setOnClickListener(new OnClickListener(){ @Override public void onClick(View p1) { lin.startAnimation(fallingAnimation); } }); OnClickListener complication= new OnClickListener(){ @Override public void onClick(View v) { switch(v.getId()){ case R.id.easyButton: perehod(Level.EASY_START,Level.EASY_END); break; case R.id.mediumButton: perehod(Level.MEDIUM_START,Level.MEDIUM_END); break; case R.id.hardButton: perehod(Level.HARD_START,Level.HARD_END); break; } } }; easyBuutton.setOnClickListener(complication); mediumButton.setOnClickListener(complication); hardButton.setOnClickListener(complication); } void perehod(int start,int end){ Intent intent = new Intent(HomeActivity.this, MainActivity.class); intent.putExtra(Level.LEVEL_START,start); intent.putExtra(Level.LEVEL_END,end); startActivity(intent) } } 

MainActivity (second activation in which the random number is selected, subtraction and comparison):

 public class MainActivity extends Activity { //int a=0,b=100 int c,z; int s=16000; final int start = getIntent().getIntExtra(Level.LEVEL_START,0); final int end = getIntent().getIntExtra(Level.LEVEL_END, 1); TextView mainTextView, mainTextView1,time; Button one,two,three,four,five,six,seven,eight,nine,zero,delit,minus,inspection; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); regist(); soz_primer(); new CountDownTimer(16000, 1000) { public void onTick(long millisUntilFinished) { //тут пиши то, что будет выполняться каждую секунду time.setText(""+millisUntilFinished/1000); } public void onFinish() { //тут пиши то, что должно произойти когда таймер закончит работу time.setText("0"); } }.start(); final OnClickListener button=new OnClickListener(){ @Override public void onClick(View v) { String text=((Button)v).getText().toString(); mainTextView1.append(""+text); } }; one.setOnClickListener(button); two.setOnClickListener(button); three.setOnClickListener(button); four.setOnClickListener(button); five.setOnClickListener(button); six.setOnClickListener(button); seven.setOnClickListener(button); eight.setOnClickListener(button); nine.setOnClickListener(button); zero.setOnClickListener(button); minus.setOnClickListener(button); delit.setOnClickListener(new OnClickListener(){ @Override public void onClick(View p1) { mainTextView1.setText(""); } }); inspection.setOnClickListener(new OnClickListener(){ @Override public void onClick(View p1) { z=Integer.parseInt(mainTextView1.getText().toString()); if(c==z){ mainTextView1.setText(""); soz_primer(); int v=Integer.parseInt(time.getText().toString()); } else finish(); } }); } void regist(){ one=(Button)findViewById(R.id.one); two=(Button)findViewById(R.id.two); three=(Button)findViewById(R.id.three); four=(Button)findViewById(R.id.four); five=(Button)findViewById(R.id.five); six=(Button)findViewById(R.id.six); seven=(Button)findViewById(R.id.seven); eight=(Button)findViewById(R.id.eight); nine=(Button)findViewById(R.id.nine); zero=(Button)findViewById(R.id.zero); delit=(Button)findViewById(R.id.delit); minus=(Button)findViewById(R.id.minus); time=(TextView)findViewById(R.id.time); mainTextView=(TextView)findViewById(R.id.mainTextView); mainTextView1=(TextView)findViewById(R.id.mainTextView1); inspection=(Button)findViewById(R.id.inspection); } void soz_primer(){ //int random_number = start + (int) (Math.random() * end); //int random_number1 = start+ (int) (Math.random() * end); int random_number=new Random().nextInt(start)+end; int random_number1=new Random().nextInt(start)+end; mainTextView.setText(""+random_number); int i=(int)(Math.random()*2); if(i==0){ c=random_number+random_number1; mainTextView.append("+"); }else mainTextView.append("-"); mainTextView.append(""+random_number1+"="); c=random_number-random_number1; } } 

Class with keys and constants:

 public class Level { public static String LEVEL_START="level_start"; public static String LEVEL_END="level_end"; public static int EASY_START=0; public static int EASY_END=100; public static int MEDIUM_START=100; public static int MEDIUM_END=1000; public static int HARD_START=900; public static int HARD_END=10000; }