AlertDialog.Builder class not found

I'm trying to create an Alert dialog via an AndroidJavaObject, but get a ClassNotFoundException:

Code (CSharp):

1.         var title = "Title";
2.         var message = "hello";
3.         AndroidJavaClass unityPlayer = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
4.         AndroidJavaObject activity = unityPlayer.GetStatic<AndroidJavaObject>("currentActivity");
5.  
6.         AndroidJavaObject builder =
7.             new AndroidJavaObject("android.app.AlertDialog.Builder", activity);
8.         dialog = builder.Call<AndroidJavaObject>("create");
9.         dialog.Call<AndroidJavaObject>("setTitle", title);
10.         dialog.Call<AndroidJavaObject>("setMessage", message);
11.  
12.         activity.Call("runOnUiThread", new AndroidJavaRunnable(() =>
13.         {
14.             dialog.Call("show");
15.         }));

Is it a problem that Builder is a nested class?

Exception: