通过我的应用程序在Facebook上成为请求好友

Question

我正在使用Facebook开发人员从我的应用发出请求添加好友 此链接：https://developers.facebook.com/docs/graph-api/reference/v3.2/user/friendrequests

这是我的代码：

private void evenHandler() {
    btnAdd.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Intent i = new Intent(LoginActivity.this, SignUpActivity.class);
            //  startActivity(i);
            new GraphRequest(
                    AccessToken.getCurrentAccessToken(),
                    "/me/friendrequests",
                    null,
                    HttpMethod.GET,
                    new GraphRequest.Callback() {
                        public void onCompleted(GraphResponse response) {
                            /* handle the result */
                        }
                    }
            ).executeAsync();

        }
    });

}

但是我有错误

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
                     at android.app.ActivityThread.-wrap11(Unknown Source:0)
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
                     at android.os.Handler.dispatchMessage(Handler.java:105)
                     at android.os.Looper.loop(Looper.java:164)
                     at android.app.ActivityThread.main(ActivityThread.java:6944)
                     at java.lang.reflect.Method.invoke(Native Method)
                     at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
                  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                     at com.befamous.MainActivity.evenHandler(MainActivity.java:185)
                     at com.befamous.MainActivity.onCreate(MainActivity.java:43)
                     at android.app.Activity.performCreate(Activity.java:7174)
                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
                     at android.app.ActivityThread.-wrap11(Unknown Source:0) 
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
                     at android.os.Handler.dispatchMessage(Handler.java:105) 
                     at android.os.Looper.loop(Looper.java:164) 
                     at android.app.ActivityThread.main(ActivityThread.java:6944) 
                     at java.lang.reflect.Method.invoke(Native Method) 
                     at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)