Merge pull request #352 from telerik/Nasko/datagrid-clear-dragvisual

Set the Content of the DataGrid's DragVisual to null in order to prev…

Controls/Grid/Grid.UWP/View/Controls/ColumnHeaders/DataGridColumnDragControl.cs

@@ -1,9 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using Windows.UI.Xaml;
+using Windows.UI.Xaml;
 
 namespace Telerik.UI.Xaml.Controls.Grid.Primitives
 {
@@ -40,5 +35,12 @@ namespace Telerik.UI.Xaml.Controls.Grid.Primitives
                 this.SetValue(FilterGlyphVisibilityProperty, value);
             }
         }
+
+        /// <inheritdoc />
+        protected override void OnUnloaded(object sender, RoutedEventArgs e)
+        {
+            base.OnUnloaded(sender, e);
+            this.Content = null;
+        }
     }
 }