The dialog box does not appear on the MainActivity called from the AsyncTask class

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I have MainActivity class and separate AsyncTask class, which i am executing from my main activity class. I am trying to display dialog box on Main Activity layout which should be called from AsyncTask class onPreExecute method.

I am trying to implement this code, no dialog box is showing on main layout and app stops. plz help :)

here is what code looks like

MainActivity class

    public class MainActivity extends Activity implements LoginResponse  {
          public static Context context_main;
         LoginThread login = new LoginThread(context_main);

        protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

             login.execute();
}

        .
        .
        .
    }

and LoginThread Class

public class LoginThread extends  AsyncTask<Void, Void, RequestToken>  {

        .
        .
        .
    public LoginThread(Context context)
    {
        this.context= context;
    }

    protected void onPreExecute()
        {
            super.onPreExecute();
            pDialog = new ProgressDialog(context);
                pDialog.setMessage(Html.fromHtml("<b>Logging In</b><br/>Please wait..."));
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(true);
            pDialog.show();
        }

               .
               .
               .

}

this is logcat msgs

03-12 15:02:10.838: E/Error(32517): I got error
03-12 15:02:10.838: E/Error(32517): java.lang.NullPointerException
03-12 15:02:10.838: E/Error(32517):     at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
03-12 15:02:10.838: E/Error(32517):     at android.app.AlertDialog.<init>(AlertDialog.java:98)
03-12 15:02:10.838: E/Error(32517):     at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
03-12 15:02:10.838: E/Error(32517):     at pack.locationinfo.LoginThread.onPreExecute(LoginThread.java:87)
03-12 15:02:10.838: E/Error(32517):     at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)
03-12 15:02:10.838: E/Error(32517):     at android.os.AsyncTask.execute(AsyncTask.java:534)
03-12 15:02:10.838: E/Error(32517):     at pack.locationinfo.MainActivity.TwitterLogin(MainActivity.java:53)
03-12 15:02:10.838: E/Error(32517):     at java.lang.reflect.Method.invokeNative(Native Method)
03-12 15:02:10.838: E/Error(32517):     at java.lang.reflect.Method.invoke(Method.java:525)
03-12 15:02:10.838: E/Error(32517):     at android.view.View$1.onClick(View.java:3833)
03-12 15:02:10.838: E/Error(32517):     at android.view.View.performClick(View.java:4475)
03-12 15:02:10.838: E/Error(32517):     at android.view.View$PerformClick.run(View.java:18784)
03-12 15:02:10.838: E/Error(32517):     at android.os.Handler.handleCallback(Handler.java:730)
03-12 15:02:10.838: E/Error(32517):     at android.os.Handler.dispatchMessage(Handler.java:92)
03-12 15:02:10.838: E/Error(32517):     at android.os.Looper.loop(Looper.java:137)
03-12 15:02:10.838: E/Error(32517):     at android.app.ActivityThread.main(ActivityThread.java:5414)
03-12 15:02:10.838: E/Error(32517):     at java.lang.reflect.Method.invokeNative(Native Method)
03-12 15:02:10.838: E/Error(32517):     at java.lang.reflect.Method.invoke(Method.java:525)
03-12 15:02:10.838: E/Error(32517):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
03-12 15:02:10.838: E/Error(32517):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
03-12 15:02:10.838: E/Error(32517):     at dalvik.system.NativeStart.main(Native Method)
03-12 15:02:16.108: D/mali_winsys(32517): new_window_surface returns 0x3000


This line causing the crash...here, context_main is null.

LoginThread login = new LoginThread(context_main);

Now, initialize the LoginThread object inside the onCreate() method as below...The crash is happening because, the Context of MainActivity isn't properly assigned.

    LoginThread login;

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    login = new LoginThread(this);

    }