Moved Overlapped stuff to ApplicationOverlap static class. Fixed nullable warnings.

Terminal.Gui/Application/Application.Overlapped.cs

@@ -106,10 +106,8 @@ public static class ApplicationOverlapped
     /// <returns></returns>
     public static bool MoveToOverlappedChild (Toplevel? top)
     {
-        if (top is null)
-        {
-            return false;
-        }
+        ArgumentNullException.ThrowIfNull (top);
+
         if (top.Visible && OverlappedTop is { } && Application.Current?.Modal == false)
         {
             lock (Application.TopLevels)

UnitTests/Views/OverlappedTests.cs

@@ -527,7 +527,7 @@ public class OverlappedTests
     [Fact]
     public void MoveToOverlappedChild_Throw_NullReferenceException_Passing_Null_Parameter ()
     {
-        Assert.Throws<NullReferenceException> (delegate { ApplicationOverlapped.MoveToOverlappedChild (null); });
+        Assert.Throws<ArgumentNullException> (delegate { ApplicationOverlapped.MoveToOverlappedChild (null); });
     }
 
     [Fact]