title: App unfortunately Crashing
body: <pre><code>//I'm new to programming and android studio
  package com.example.calc;
  
  import android.os.Bundle;
  import android.support.v7.app.AppCompatActivity;
  import android.text.TextUtils;
  import android.view.View;
  import android.widget.Button;
  import android.widget.EditText;
  
  import static com.example.calc.R.id.editText;
  
  public class MainActivity extends AppCompatActivity implements View.OnClickListener {
  
  EditText editTexts = (EditText) findViewById(editText);
  double a = 0.0;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  
                                                     //Buttons to take input and display
      Button button1 = (Button) findViewById(R.id.button1);
      Button button2 = (Button) findViewById(R.id.button2);
      Button button3 = (Button) findViewById(R.id.button3);
      Button button4 = (Button) findViewById(R.id.button4);
      Button button5 = (Button) findViewById(R.id.button5);
      Button button6 = (Button) findViewById(R.id.button6);
  
                                            //using to make the buttons react
      button1.setOnClickListener(this);
      button2.setOnClickListener(this);
      button3.setOnClickListener(this);
      button4.setOnClickListener(this);
      button5.setOnClickListener(this);
      button6.setOnClickListener(this);
  }
      @Override
      public void onClick (View v){
  
          double res;     //Display and taking input from same textfield
  
          if (TextUtils.isEmpty(editTexts.getText().toString())) {
              return;
          }
  
          res = Double.parseDouble(editTexts.getText().toString());
  
          switch (v.getId()) {
              case R.id.button1:
                  a += res;
                  editTexts.setText(" ");
                  break;
              case R.id.button2:
                  a -= res;
                  editTexts.setText(" ");
                  break;
              case R.id.button3:
                  a *= res;
                  editTexts.setText(" ");
                  break;
              case R.id.button4:
                  a /= res;
                  editTexts.setText(" ");
                  break;
              case R.id.button5:
                  editTexts.setText((int) a);
                  break;
              case R.id.button6:
                  a = 0.0;
                  editTexts.setText(" ");
                  break;
  
          }
  
      }
  }
  </code></pre>
  
  <p>Unfortunately, App crashed.
  Gradle is building but crashing as soon as opening.
  How to resolve this error??</p>
  
  <pre><code>logCat:
      06-24 22:36:42.926 4983-4983/? E/libprocessgroup: failed to make and chown 
      /acct/uid_10058: Read-only file system
      06-24 22:36:42.926 4983-4983/? W/Zygote: createProcessGroup failed, kernel 
      missing CONFIG_CGROUP_CPUACCT?
      06-24 22:36:42.930 4983-4983/? I/art: Not late-enabling -Xcheck:jni (already 
      on)
      06-24 22:36:43.400 4983-4983/com.example.calc I/InstantRun: starting instant 
      run server: is main process
      06-24 22:36:43.626 4983-4983/com.example.calc W/art: Verification of boolean 
  
    android.support.v7.app.AppCompatDelegateImplV9.initializePanelContent(android.support.v7.app.AppCompatDelegateImplV9$PanelFeatureState) took 139.729ms
  06-24 22:36:43.741 4983-4983/com.example.calc W/art: Verification of 
      android.support.v7.view.ActionMode 
  
  
   android.support.v7.app.AppCompatDelegateImplV9.startSupportActionModeFromWindow(android.support.v7.view.ActionMode$Callback) took 109.701ms
  06-24 22:36:43.743 4983-4983/com.example.calc D/AndroidRuntime: Shutting down VM
  
  
                                                                  --------- beginning of crash
      06-24 22:36:43.746 4983-4983/com.example.calc E/AndroidRuntime: FATAL 
      EXCEPTION: main
                                                                  Process: 
      com.example.calc, PID: 4983
  
      java.lang.RuntimeException: Unable to instantiate activity 
      ComponentInfo{com.example.calc/com.example.calc.MainActivity}: 
      java.lang.NullPointerException: Attempt to invoke virtual method 
       'android.view.Window$Callback android.view.Window.getCallback()' on a null 
       object reference
                                                                      at 
  
      android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
                                                                       at 
       android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
                                                                      at 
        android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                                      at 
      android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                                                                      at 
      android.os.Handler.dispatchMessage(Handler.java:102)
                                                                      at 
      android.os.Looper.loop(Looper.java:135)
                                                                      at 
      android.app.ActivityThread.main(ActivityThread.java:5254)
                                                                      at 
       java.lang.reflect.Method.invoke(Native Method)
                                                                      at 
      java.lang.reflect.Method.invoke(Method.java:372)
                                                                      at 
  
  
  
  
   com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                                                                      at 
       com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
                                                                   Caused by: 
       java.lang.NullPointerException: Attempt to invoke virtual method 
      'android.view.Window$Callback android.view.Window.getCallback()' on a null 
      object reference
                                                                      at 
       android.support.v7.app.AppCompatDelegateImplBase.&lt;init&gt;
      (AppCompatDelegateImplBase.java:118)
                                                                      at 
       android.support.v7.app.AppCompatDelegateImplV9.&lt;init&gt;
      (AppCompatDelegateImplV9.java:152)
                                                                      at 
      android.support.v7.app.AppCompatDelegateImplV11.&lt;init&gt;
      (AppCompatDelegateImplV11.java:29)
                                                                      at 
      android.support.v7.app.AppCompatDelegateImplV14.&lt;init&gt;
      (AppCompatDelegateImplV14.java:53)
                                                                      at 
       android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:204)
                                                                      at 
       android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:184)
                                                                      at 
  
  
   android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:518
      )
                                                                      at 
  
    android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:189
      )
                                                                      at 
      com.example.calc.MainActivity.&lt;init&gt;(MainActivity.java:15)
                                                                      at 
      java.lang.reflect.Constructor.newInstance(Native Method)
                                                                      at 
       java.lang.Class.newInstance(Class.java:1606)
                                                                      at 
       android.app.Instrumentation.newActivity(Instrumentation.java:1066)
                                                                      at 
       android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
                                                                      at 
        android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
                                                                      at 
       android.app.ActivityThread.access$800(ActivityThread.java:151) 
                                                                      at 
       android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
                                                                      at 
        android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                      at 
        android.os.Looper.loop(Looper.java:135) 
                                                                      at 
       android.app.ActivityThread.main(ActivityThread.java:5254) 
                                                                      at 
       java.lang.reflect.Method.invoke(Native Method) 
                                                                      at 
       java.lang.reflect.Method.invoke(Method.java:372) 
                                                                      at 
  
  
  
  
   com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
                                                                      at 
      com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 
  </code></pre>