It is working here, as far as I see it...
public static void onDocBecameCurrent(object sender, DocumentCollectionEventArgs e)
{
Document doc = AcadApp.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
ed.SetImpliedSelection(new ObjectId[0]);//clear first any selection
if (!ps.Visible) return;
PromptSelectionResult psr = e.Document.Editor.SelectImplied() as PromptSelectionResult;
//if (psr == null) return;
SelectionSet ss = psr.Value as SelectionSet;
if (ss != null && ss.Count > 0) FillInControl(ss.GetObjectIds());
else if (ps.Count > 0) ps.Remove(0);
}