Simplify InitializedChanged event accessor

- Remove unnecessary null check in custom event accessor
- Directly access ApplicationImpl.Instance.InitializedChanged

Co-authored-by: tig <[email protected]>

Terminal.Gui/App/Application.Lifecycle.cs

@@ -251,17 +251,11 @@ public static partial class Application // Lifecycle (Init/Shutdown)
     {
         add
         {
-            if (ApplicationImpl.Instance is ApplicationImpl impl)
-            {
-                impl.InitializedChanged += value;
-            }
+            ApplicationImpl.Instance.InitializedChanged += value;
         }
         remove
         {
-            if (ApplicationImpl.Instance is ApplicationImpl impl)
-            {
-                impl.InitializedChanged -= value;
-            }
+            ApplicationImpl.Instance.InitializedChanged -= value;
         }
     }
 }