これは私が使用しているクラスです
public class ContactsXmpp extends SherlockFragmentActivity {
private static Context ctx;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.contacts_xmpp_sip);
    ctx = this;
}   
このメソッドから asynctask を呼び出すとエラーが発生します。これがエラーです 
No enclosing instance of type ContactsXmpp is accessible. Must qualify the allocation with an enclosing instance of type ContactsXmpp (e.g. x.new A() where x is an instance of ContactsXmpp).
private static void alert( String str, final String name ) {
    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(ctx);       
    alertDialogBuilder.setMessage(str + ": " + name);
    alertDialogBuilder.setCancelable(false)
    .setPositiveButton("Yes",new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog,int id) {                 
            Subscription(name);          
            new ColorsXMPP_AfterLogin().execute();     ///** error getting here..                                
        }
    }); 
    alertDialogBuilder.setNegativeButton("No", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            unSubscribe(name);
        }           
    });
    AlertDialog alertDialog = alertDialogBuilder.create();       
    alertDialog.show();         
}    
ここで非同期タスク
public class ColorsXMPP_AfterLogin extends AsyncTask<AfterLogging, Void, Void> {
    private ProgressDialog _dialog;
    @Override
    protected void onPreExecute() {          
        super.onPreExecute();            
        Log.e(TAG, " GmailXMPP_AfterLogin onPreExecute" );
    }
    @Override
    protected void onPostExecute(Void feed) {
        //ProgressBar_hide();       
        _dialog.dismiss();
        Log.e(TAG, " GmailXMPP_AfterLogin onPostExecute" );        
    }
    @Override
    protected Void doInBackground(AfterLogging... arg0) {
        Log.e(TAG, " GmailXMPP_AfterLogin doInBackground" );
        return null;
    }
 }