title: Delete button in calculator
body: <p>I'm creating a calculator, I made a delete button but when I run the app, it crashes and stops.
  This is my java code:</p>
  
  <p>Help me please.</p>
  
  <pre><code>       private static String removeLastChar(String str) {
         return str.substring(0,str.length()-1);
  </code></pre>
  
  <p>and this </p>
  
  <pre><code>        bdelete.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
              editText.setText(removeLastChar(editText.getText().toString()));
          }
         });
  </code></pre>
  
  <p>here's my log</p>
  
  <pre><code>java.lang.RuntimeException: Unable to start activity   ComponentInfo{player.android.com.layout/player.android.com.layout.MyActivity}:  java.lang.NullPointerException
           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
           at android.app.ActivityThread.access$600(ActivityThread.java:141)
           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
           at android.os.Handler.dispatchMessage(Handler.java:99)
           at android.os.Looper.loop(Looper.java:137)
           at android.app.ActivityThread.main(ActivityThread.java:5103)
           at java.lang.reflect.Method.invokeNative(Native Method)
           at java.lang.reflect.Method.invoke(Method.java:525)
           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
           at dalvik.system.NativeStart.main(Native Method)
   Caused by: java.lang.NullPointerException
           at player.android.com.layout.MyActivity.onCreate(MyActivity.java:68)
           at android.app.Activity.performCreate(Activity.java:5133)
           at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
           at android.app.ActivityThread.access$600(ActivityThread.java:141)
           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
           at android.os.Handler.dispatchMessage(Handler.java:99)
           at android.os.Looper.loop(Looper.java:137)
           at android.app.ActivityThread.main(ActivityThread.java:5103)
           at java.lang.reflect.Method.invokeNative(Native Method)
           at java.lang.reflect.Method.invoke(Method.java:525)
           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
           at dalvik.system.NativeStart.main(Native Method)
  </code></pre>