unity Exception:JNI:Init'd AndroidJavaObject with null prt!

when my program run at 5 line in Unity Test.cs, " AndroidJavaObject jo = jc.CallStatic("instance");", it throw out the exception as described and end up.

Exception:JNI:Init'd AndroidJavaObject with null prt! at UnityEngine.AndroidJavaObject..ctor(IntPtr jobject)(0x00000)in:0

 MainActivity.java
 
 public class MainActivity extends UnityPlayerActivity {
 
  private static MainActivity instance;
  private static Context context;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      MainActivity.instance = this;
      context = getApplicationContext();
  }
  
  public void InstallAPK(String path)
  {
      Log.d("debug","InstallAPK path"+path);
      File apkFile = new File(path);
      
      if(apkFile.exists())
      {
          Log.d("debug","InstallAPK!");
          Intent intent = new Intent(Intent.ACTION_VIEW);
          intent.setDataAndType(Uri.fromFile(apkFile),"application/vnd.android.package-archive");
          context.startActivity(intent);
      }
      else
      {
          Log.d("debug","fiald file!");
      }
  }
  
  public static MainActivity instance()
  {
      return instance;
  }
 }


 AndroidManifest.xml
 
      <application
    android:name="com.talkingsdk.sdk.SdkMainApplication"
    android:icon="@drawable/app_icon"
    android:label="@string/app_name"
    android:debuggable="false">
    <activity android:configChanges="locale|mcc|mnc|touchscreen|keyboard|keyboardHidden|navigation|orientation|screenLayout|uiMode|screenSize|smallestScreenSize|fontScale" 
      android:label="@string/app_name" 
      android:launchMode="singleTask" 
      android:name="com.unity3d.player.UnityPlayerProxyActivity" 
      android:screenOrientation="landscape"/>
    <activity android:configChanges="locale|mcc|mnc|touchscreen|keyboard|keyboardHidden|navigation|orientation|screenLayout|uiMode|screenSize|smallestScreenSize|fontScale" 
      android:label="@string/app_name" 
      android:launchMode="singleTask" 
      android:name="com.unity3d.player.UnityPlayerActivity" 
      android:screenOrientation="landscape"/>
    <activity android:configChanges="locale|mcc|mnc|touchscreen|keyboard|keyboardHidden|navigation|orientation|screenLayout|uiMode|screenSize|smallestScreenSize|fontScale" 
      android:label="@string/app_name" 
      android:name="com.talkingsdk.umeng.GamePushActivity">
          <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
          </intent-filter>
    </activity>
      <activity
          android:name="com.cmd.kz3d.qihu.MainActivity"
          android:label="@string/app_name" >
      </activity>

 Unity Test.cs
 
  void OnGUI()
  {
      if(GUI.Button(new Rect(10,250,450,100),"Install"))
      {
          AndroidJavaClass jc = new AndroidJavaClass("com.cmd.kz3d.qihu.MainActivity");
     AndroidJavaObject jo = jc.CallStatic<AndroidJavaObject>("instance");
          string path = Application.persistentDataPath+"/test.apk";
          Debug.Log(path);
          if(!File.Exists(path))
          {
               Debug.Log("not found");
          }
          //jc.Call("InstallAPK", path);
          jc.Call("InstallAPK", path);
      }
  }