play-games-plugin-for-unity Authentication issue

I have downloaded the google play service form https://github.com/playgameservices/play-games-plugin-for-unity. i have followed the tutorial on github and implemented it. When i click on log in button nothing happened. Here is the code.

 using UnityEngine;
 using System.Collections;
 using GooglePlayGames;
 using UnityEngine.SocialPlatforms;
 
 public class testGPGMenu : MonoBehaviour
 {
 
 public GUISkin skin;
 public int incrementalCount = 5;
 
 //leaderboard strings
 private string leaderboard = "C----------------g";
 //achievement strings
 private string achievement = "C----------------Q";
 private string incremental = "C----------------g";
 
 // Use this for initialization
 void Start()
 {
     PlayGamesPlatform.Activate();
 }
 
 // Update is called once per frame
 public void OnGUI()
 {
     GUI.skin = skin;
     skin.button.fixedWidth = Screen.width - 25;
     skin.textField.fixedWidth = Screen.width - 25;
     GUILayout.BeginArea(new Rect(0, 0, Screen.width, Screen.height));
 
     GUILayout.BeginVertical("box");
 
     GUILayout.Label("Official Google Play Games Services");
 
     GUILayout.Space(20);
 
     //Share Status
     if (GUILayout.Button("Log In"))
     {
         Social.localUser.Authenticate((bool success) =>
                                       {
             if (success)
             {
                 Debug.Log("You've successfully logged in");
             }
             else
             {
                 Debug.Log("Login failed for some reason");
             }
         });
     }
 
     GUILayout.Space(20);
 
     //Achievement
     if (GUILayout.Button("Unlock Achievement"))
     {
         if (Social.localUser.authenticated)
         {
             Social.ReportProgress(achievement, 100.0f, (bool success) =>
                                   {
                 if (success)
                 {
                     Debug.Log("You've successfully logged in");
                 }
                 else
                 {
                     Debug.Log("Login failed for some reason");
                 }
             });
         }
     }
 
     GUILayout.Space(20);
 
     //Incremental Achievement
     if (GUILayout.Button("Press " + incrementalCount + "more times to unlock incremental acheivment"))
     {
         if (Social.localUser.authenticated)
         {
             ((PlayGamesPlatform)Social.Active).IncrementAchievement(incremental, 5, (bool success) =>
                                                                     {
                 //The achievement unlocked successfully
             });
         }
     }
 
     GUILayout.Space(20);
 
     //Leaderboard
     if (GUILayout.Button("Post your 5000 points to the leaderboard"))
     {
         if (Social.localUser.authenticated)
         {
             Social.ReportScore(5000, leaderboard, (bool success) =>
                                {
                 if (success)
                 {
                     ((PlayGamesPlatform)Social.Active).ShowLeaderboardUI(leaderboard);
                 }
                 else
                 {
                     //Debug.Log("Login failed for some reason");
                 }
             });
         }
     }
 
     GUILayout.Space(20);
 
     // Show Leaderboard
     if (GUILayout.Button("Show Leaderboard"))
     {
         Social.ShowLeaderboardUI();
     }
 
     GUILayout.Space(20);
 
     //Show Specific Leaderboard
     if (GUILayout.Button("Show Specific Leaderboard"))
     {
         ((PlayGamesPlatform)Social.Active).ShowLeaderboardUI(leaderboard);
     }
 
     GUILayout.Space(20);
 
     //Show Achievments
     if (GUILayout.Button("Show Achievments"))
     {
         Social.ShowAchievementsUI();
     }
 
     GUILayout.Space(20);
 
     //Sign Out
     if (GUILayout.Button("Sign Out"))
     {
         ((PlayGamesPlatform)Social.Active).SignOut();
     }
 
     GUILayout.EndVertical();
     GUILayout.EndArea();
 }
 
 }

Here is what i get in logcat 07-22 15:43:00.678: I/Unity(1222): NullReferenceException: Object reference not set to an instance of an object 07-22 15:43:00.678: I/Unity(1222): at GooglePlayGames.Android.AndroidClient+c_AnonStorey0.<>m_1 () [0x00000] in :0 07-22 15:43:00.678: I/Unity(1222): at UnityEngine.AndroidJavaRunnableProxy.run () [0x00000] in :0 07-22 15:43:00.678: I/Unity(1222): at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in :0 07-22 15:43:00.678: I/Unity(1222): Rethrow as TargetInvocationException: UnityEngine.AndroidJavaRunnableProxy.run() 07-22 15:43:00.678: I/Unity(1222): at UnityEngine.AndroidJavaProxy.Invoke (System.String methodName, System.Object[] args) [0x00000] in :0 07-22 15:43:00.678: I/Unity(1222): at UnityEngine.AndroidJavaProxy.Invoke (System.String methodName, UnityEngine.AndroidJavaObject[] javaArgs) [0x00000] in :0 07-22 15:43:00.678: I/Unity(1222): at UnityEngine._AndroidJNIHelper.InvokeJavaProxyMethod (UnityEngine.AndroidJavaProxy proxy, IntPtr jmethodName, IntPtr jargs) [0x00000] in :0