Fixed nullability issues

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -354,7 +354,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="eventSource">The control that raised the event.</param>
         /// <returns>The container or null if the event did not originate in a container.</returns>
-        protected IControl? GetContainerFromEventSource(IInteractive eventSource)
+        protected IControl? GetContainerFromEventSource(IInteractive? eventSource)
         {
             for (var current = eventSource as IVisual; current != null; current = current.VisualParent)
             {
@@ -666,7 +666,7 @@ namespace Avalonia.Controls.Primitives
         /// false.
         /// </returns>
         protected bool UpdateSelectionFromEventSource(
-            IInteractive eventSource,
+            IInteractive? eventSource,
             bool select = true,
             bool rangeModifier = false,
             bool toggleModifier = false,