whenever run the projects emulator always gets force to close
<pre><code>08-09 19:47:17.454: E/AndroidRuntime(941): FATAL EXCEPTION: main
08-09 19:47:17.454: E/AndroidRuntime(941): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sample/com.example.sample.MainActivity}: java.lang.NullPointerException
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.os.Handler.dispatchMessage(Handler.java:99)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.os.Looper.loop(Looper.java:123)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread.main(ActivityThread.java:3683)
08-09 19:47:17.454: E/AndroidRuntime(941):  at java.lang.reflect.Method.invokeNative(Native Method)
08-09 19:47:17.454: E/AndroidRuntime(941):  at java.lang.reflect.Method.invoke(Method.java:507)
08-09 19:47:17.454: E/AndroidRuntime(941):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-09 19:47:17.454: E/AndroidRuntime(941):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
08-09 19:47:17.454: E/AndroidRuntime(941):  at dalvik.system.NativeStart.main(Native Method)
08-09 19:47:17.454: E/AndroidRuntime(941): Caused by: java.lang.NullPointerException
08-09 19:47:17.454: E/AndroidRuntime(941):  at com.example.sample.MainActivity.onCreate(MainActivity.java:21)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-09 19:47:17.454: E/AndroidRuntime(941):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
08-09 19:47:17.454: E/AndroidRuntime(941):  ... 11 more
</code></pre>
