新浪sdk 调用api报错 ，求解

public void onClick(View v) {
                Weibo w=Weibo.getInstance();
                String url="https://api.weibo.com/2/users/show.json";
                WeiboParameters wp=new WeiboParameters();
                wp.add("source",Weibo.getAppKey());
                wp.add("uid",user_id);
                wp.add("access_token",w.getAccessToken().getToken());
                try {
                    String s=w.request(callbackActivity.this, url, wp, "GET",w.getAccessToken());
                    System.out.println("-----"+s);
                } catch (WeiboException e) {
                    e.printStackTrace();
                }FATAL EXCEPTION: main
java.lang.NullPointerException
atjava.net.URLEncoder.encode(URLEncoder.java:53)
atcom.weibo.net.Utility.encodeUrl(Utility.java:198)
atcom.weibo.net.Utility.openUrl(Utility.java:301)
at com.weibo.net.Utility.openUrl(Utility.java:286)
atcom.weibo.net.Weibo.request(Weibo.java:149)
atweibo.OAuth.ui.callbackActivity$4.onClick(callbackActivity.java:145)
atandroid.view.View.performClick(View.java:2533)
tandroid.view.View$PerformClick.run(View.java:9320)
atandroid.os.Handler.handleCallback(Handler.java:587)
atandroid.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:150)
atandroid.app.ActivityThread.main(ActivityThread.java:4385)
atjava.lang.reflect.Method.invokeNative(Native Method)
atjava.lang.reflect.Method.invoke(Method.java:507)
atcom.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849)
atcom.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
at dalvik.system.NativeStart.main(NativeMethod)有人知道怎么解决吗？