对话框片段中缺少标题边框

Question

我多次使用对话框片段，标题边框总是出现，但这次它没有

这是我的代码

public class DailogBox extends DialogFragment {



@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {


    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());


    LayoutInflater inflater=getActivity().getLayoutInflater();
    builder.setTitle("title");

    builder.setMessage("message");
    AlertDialog alertdialog = builder.create();


    return alertdialog;
}
}

这是我的style.xml

 <!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
</style>

我不确定主题是否与问题有关

这是我得到的输出

http://oi60.tinypic.com/64gfx1.jpg