android – dialog.getWindow()在创建对话框后返回null

@Override
protected Dialog onCreateDialog(int id) {
    Dialog dialog = null;
    switch(id) {
    case DIALOG_ID:
        LayoutInflater inflater = getLayoutInflater();
        View InfoLayout = inflater.inflate(R.layout.info_dialog,null);
        AlertDialog.builder b = new AlertDialog.builder(MyActivity.this)
                .setView(InfoLayout );
        iniTinfoDialog(dialog);
        return dialog;
...

private void iniTinfoDialog(Dialog dialog) {
    //this line has the null pointer 
    WindowManager.LayoutParams lp = dialog.getWindow().getAttributes();

为什么dialog.getWindow()在这种情况下返回null？