XML : how can i solve this errors on android? [duplicate]

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• What is a Null Pointer Exception, and how do I fix it? 12 answers

  12-18 15:47:41.905: E/AndroidRuntime(6520): FATAL EXCEPTION: main  12-18 15:47:41.905: E/AndroidRuntime(6520): java.lang.NullPointerException  12-18 15:47:41.905: E/AndroidRuntime(6520):     at com.example.myproject.MainActivity$4.onClick(MainActivity.java:91)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.view.View.performClick(View.java:4084)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.view.View$PerformClick.run(View.java:16966)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.os.Handler.handleCallback(Handler.java:615)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.os.Handler.dispatchMessage(Handler.java:92)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.os.Looper.loop(Looper.java:137)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at android.app.ActivityThread.main(ActivityThread.java:4745)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at java.lang.reflect.Method.invokeNative(Native Method)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at java.lang.reflect.Method.invoke(Method.java:511)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)  12-18 15:47:41.905: E/AndroidRuntime(6520):     at dalvik.system.NativeStart.main(Native Method)