Fixed timeout bug

Terminal.Gui/Application/Application.Run.cs

@@ -454,7 +454,10 @@ public static partial class Application // Run (Begin, Run, End, Stop)
     ///     reset, repeating the invocation. If it returns false, the timeout will stop and be removed. The returned value is a
     ///     token that can be used to stop the timeout by calling <see cref="RemoveTimeout(object)"/>.
     /// </remarks>
-    public static object AddTimeout (TimeSpan time, Func<bool> callback) { return MainLoop!.AddTimeout (time, callback); }
+    public static object? AddTimeout (TimeSpan time, Func<bool> callback)
+    {
+        return MainLoop?.AddTimeout (time, callback) ?? null;
+    }
 
     /// <summary>Removes a previously scheduled timeout</summary>
     /// <remarks>The token parameter is the value returned by <see cref="AddTimeout"/>.</remarks>