Removed C# 8.0 feature usage

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -14,8 +14,8 @@ namespace Avalonia.Controls.Platform
     public class DefaultMenuInteractionHandler : IMenuInteractionHandler
     {
         private readonly bool _isContextMenu;
-        private IDisposable? _inputManagerSubscription;
-        private IRenderRoot? _root;
+        private IDisposable _inputManagerSubscription;
+        private IRenderRoot _root;
 
         public DefaultMenuInteractionHandler(bool isContextMenu)
             : this(isContextMenu, Input.InputManager.Instance, DefaultDelayRun)
@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Platform
 
         public DefaultMenuInteractionHandler(
             bool isContextMenu,
-            IInputManager? inputManager,
+            IInputManager inputManager,
             Action<Action, TimeSpan> delayRun)
         {
             delayRun = delayRun ?? throw new ArgumentNullException(nameof(delayRun));
@@ -94,7 +94,7 @@ namespace Avalonia.Controls.Platform
                 root.Deactivated -= WindowDeactivated;
             }
 
-            _inputManagerSubscription!.Dispose();
+            _inputManagerSubscription.Dispose();
 
             Menu = null;
             _root = null;
@@ -102,9 +102,9 @@ namespace Avalonia.Controls.Platform
 
         protected Action<Action, TimeSpan> DelayRun { get; }
 
-        protected IInputManager? InputManager { get; }
+        protected IInputManager InputManager { get; }
 
-        protected IMenu? Menu { get; private set; }
+        protected IMenu Menu { get; private set; }
 
         protected static TimeSpan MenuShowDelay { get; } = TimeSpan.FromMilliseconds(400);
 
@@ -133,7 +133,7 @@ namespace Avalonia.Controls.Platform
             KeyDown(GetMenuItem(e.Source as IControl), e);
         }
 
-        protected internal virtual void KeyDown(IMenuItem? item, KeyEventArgs e)
+        protected internal virtual void KeyDown(IMenuItem item, KeyEventArgs e)
         {
             switch (e.Key)
             {
@@ -202,7 +202,7 @@ namespace Avalonia.Controls.Platform
                     }
                     else
                     {
-                        Menu!.Close();
+                        Menu.Close();
                     }
 
                     e.Handled = true;
@@ -215,7 +215,7 @@ namespace Avalonia.Controls.Platform
                     {
                         if (item == null && _isContextMenu)
                         {
-                            if (Menu!.MoveSelection(direction.Value, true) == true)
+                            if (Menu.MoveSelection(direction.Value, true) == true)
                             {
                                 e.Handled = true;
                             }
@@ -410,7 +410,7 @@ namespace Avalonia.Controls.Platform
 
         protected void CloseMenu(IMenuItem item)
         {
-            var current = (IMenuElement?)item;
+            var current = (IMenuElement)item;
 
             while (current != null && !(current is IMenu))
             {
@@ -458,7 +458,7 @@ namespace Avalonia.Controls.Platform
 
         protected void SelectItemAndAncestors(IMenuItem item)
         {
-            var current = (IMenuItem?)item;
+            var current = item;
 
             while (current?.Parent != null)
             {
@@ -467,7 +467,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected static IMenuItem? GetMenuItem(IControl? item)
+        protected static IMenuItem GetMenuItem(IControl item)
         {
             while (true)
             {