when i click to go on another activity i get error java.lang.RuntimeException: Unable to start activity ComponentInfo {…}: java.lang.NullPointerException

[shivam1692]

This my java file

import android.net.wifi.WifiConfiguration.Status;
import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.app.TabActivity;
import android.content.ComponentName;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class Tab_login extends Activity {

Boolean isConnectionExist=false;
WIfiInternetConnectionDetector cd;

Context context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_tab_login);
    cd= new WIfiInternetConnectionDetector(getApplicationContext());
    isConnectionExist = cd.checkMobileInternetcon();

    if(isConnectionExist)
    {

    Button register= (Button)findViewById(R.id.button2);
    register.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
        Intent it= new Intent(Tab_login.this,Registration.class);
        startActivity(it);
        }
    });
    Button bt= (Button)findViewById(R.id.button3);
    bt.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub

            //startActivityFromChild(Login.class, Intent it = new Intent(), requestCode)
            Intent i = new Intent(Tab_login.this,Login.class);
            startActivity(i);
        }
    });
}

    else
    {   
        AlertDialog.Builder wifi = new AlertDialog.Builder(context);

        wifi.setTitle("Your Device is not connected with Wi-Fi");
        wifi.setMessage("To change settings Kindly tap on the Button Change");
        wifi.setIcon(R.drawable.cancel_d);
        wifi.setPositiveButton("Change", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub



            }
        });

        wifi.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                Toast.makeText(getApplicationContext(),"Switch on your Wi-Fi first", Toast.LENGTH_LONG).show();
            Intent it = new Intent(Tab_login.this,MainActivity.class);
            startActivity(it);

            }
        });


    }
}


public void openWifisetting()
{
    final Intent it = new Intent(Intent.ACTION_MAIN,null);
    it.addCategory(Intent.CATEGORY_LAUNCHER);
    final ComponentName cn = new ComponentName("com.android.settings", "com.android.settings.wifi.WifiSettings");
    it.setComponent(cn);
    it.setFlags(it.FLAG_ACTIVITY_NEW_TASK);
    startActivity(it);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

/* (non-Javadoc)
 * @see android.app.Activity#onStart()
 */
@Override
protected void onStart() {
    // TODO Auto-generated method stub


    super.onStart();
}

}

here is logcat

08-02 06:06:41.397: E/AndroidRuntime(2334): FATAL EXCEPTION: main
08-02 06:06:41.397: E/AndroidRuntime(2334): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.xxxi/com.example.xxx.Tab}: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.xxx/com.example.xxx.Tab_login}: java.lang.NullPointerException
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.os.Handler.dispatchMessage(Handler.java:99)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.os.Looper.loop(Looper.java:137)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.main(ActivityThread.java:5103)
08-02 06:06:41.397: E/AndroidRuntime(2334): at java.lang.reflect.Method.invokeNative(Native Method)
08-02 06:06:41.397: E/AndroidRuntime(2334): at java.lang.reflect.Method.invoke(Method.java:525)
08-02 06:06:41.397: E/AndroidRuntime(2334): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
08-02 06:06:41.397: E/AndroidRuntime(2334): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-02 06:06:41.397: E/AndroidRuntime(2334): at dalvik.system.NativeStart.main(Native Method)
08-02 06:06:41.397: E/AndroidRuntime(2334): Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.xxx/com.example.xxx.Tab_login}: java.lang.NullPointerException
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.startActivityNow(ActivityThread.java:2054)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:135)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:347)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:749)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.widget.TabHost.setCurrentTab(TabHost.java:413)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.widget.TabHost.addTab(TabHost.java:240)
08-02 06:06:41.397: E/AndroidRuntime(2334): at com.example.xxx.Tab.onCreate(Tab.java:41)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.Activity.performCreate(Activity.java:5133)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
08-02 06:06:41.397: E/AndroidRuntime(2334): ... 11 more
08-02 06:06:41.397: E/AndroidRuntime(2334): Caused by: java.lang.NullPointerException
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.AlertDialog$Builder.(AlertDialog.java:360)
08-02 06:06:41.397: E/AndroidRuntime(2334): at com.example.xxx.Tab_login.onCreate(Tab_login.java:62)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.Activity.performCreate(Activity.java:5133)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
08-02 06:06:41.397: E/AndroidRuntime(2334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
08-02 06:06:41.397: E/AndroidRuntime(2334): ... 21 more