Added nullability checks

Author: jsuarezruiz

src/Platform.Handlers/src/Xamarin.Platform.Handlers/Handlers/CheckBox/CheckBoxHandler.iOS.cs

@@ -64,11 +64,10 @@ public override Size GetDesiredSize(double widthConstraint, double heightConstra
 			return size;
 		}
 
-		void OnCheckedChanged(object sender, EventArgs e)
+		void OnCheckedChanged(object? sender, EventArgs e)
 		{
-			if (VirtualView != null)
+			if (sender is NativeCheckBox nativeView && VirtualView != null)
 			{
-				var nativeView = (NativeCheckBox)sender;
 				VirtualView.IsChecked = nativeView.IsChecked;
 			}
 		}

src/Platform.Handlers/src/Xamarin.Platform.Handlers/Platform/iOS/NativeCheckBox.cs

@@ -45,10 +45,10 @@ public NativeCheckBox()
 			TouchUpInside += OnTouchUpInside;
 		}
 
-		void OnTouchUpInside(object sender, EventArgs e)
+		void OnTouchUpInside(object? sender, EventArgs e)
 		{
 			IsChecked = !IsChecked;
-			CheckedChanged?.Invoke(this, null);
+			CheckedChanged?.Invoke(this, EventArgs.Empty);
 		}
 
 		public bool IsChecked