Extra space under search results after each focusing in searchbar

Hello I added UISearchbar in navigation bar of collection view controller. Search wroks fine. But when search result pop-up window is closed, and I begib search once more space between first cell of results and window header is increasing. And it's happaning each time (screens added). When I return backward from collection view screen to previous screen and onward everythings starts from begingin. First search is ok, second has a space and so on.

adding searchbar:

void setupGridSearch ()
        {
            if (gridCollection == null)
                throw new NullReferenceException ("field gridCollection reference is null");

            search = new ReportGridSearch<ReportGridCollection> (gridCollection);

            searchBar = new UISearchBar (new RectangleF (0, 0, 300, 50)) {
                Placeholder = Constants.ReportGridSearchBarPlaceholderText,
                ShowsBookmarkButton = false,
                ShowsCancelButton = false,
                ShowsSearchResultsButton = false,
            };

            var srd = new PeportCollectionViewSearchResultDelegate (this, search);
            srd.RowSelectedEvent += searchResultRowSelected_RowSelectedEvent;

            searchController = new UISearchDisplayController (searchBar, this) {
                SearchResultsSource = new ReportGridSearchResultSource (search),
                Delegate = new ReportGridSearchDelegate (gridCollection, search),
                SearchResultsDelegate = srd,
                SearchResultsTitle = Constants.ReportGridSearchResultsTitle,
            };


            UIBarButtonItem sbi = new UIBarButtonItem (searchBar);
            NavigationItem.RightBarButtonItem = sbi;
        }

search result source:

public class ReportGridSearchResultSource : UITableViewSource
        {
            List<string> resultList;
            ReportGridSearch<ReportGridCollection> search;

            const string cellIdentifier = "TableCell";
            const string headerIdentifier = "TableHeader";

            public ReportGridSearchResultSource (ReportGridSearch<ReportGridCollection> search)
            {
                this.search = search;
            }

            public override int RowsInSection (UITableView tableview, int section)
            {
                return search.SearchResult.Count;
            }

            public override int NumberOfSections (UITableView tableView)
            {
                return 1;
            }

            public override UITableViewCell GetCell (UITableView tableView, MonoTouch.Foundation.NSIndexPath indexPath)
            {
                var cell = tableView.DequeueReusableCell (cellIdentifier);
                // if there are no cells to reuse, create a new one
                if (cell == null)
                    cell = new UITableViewCell (UITableViewCellStyle.Default, cellIdentifier);
                cell.TextLabel.Text = search.SearchResult.ElementAt (indexPath.Row).Value;
                return cell;
            }

        }

May be somebody had same issue?