Error 1400

Hello,

When i'm closing the application eg, after 30 seconds OTL will raise an exception with error code 1400 invalid window handle. Any ideas ?

```
var
FTask1: IOmniTimedTask;
FTask2: IOmniTimedTask;

procedure DoSometing1;
begin
end;

procedure DoSometing2;
begin
end;

procedure StartTimedTasks(const Task: IOmniTask);
begin
FTask1 := Parallel.TimedTask.Every(1000).Execute(DoSometing1);
FTask2 := Parallel.TimedTask.Every(1000).Execute(DoSometing2);
FTask1.Start;
FTask2.Start;
end;

procedure TForm8.FormCreate(Sender: TObject);
begin
Parallel.Async(StartTimedTasks);
end;
```

Start with the FormCreate and StartTimedTasks flow shown in the report, then reproduce closing the application after roughly 30 seconds while both timed tasks are running. Trace where error 1400 is raised during shutdown; done means the close completes without the invalid-window-handle exception and the relevant behavior is covered by a regression test.

Written by the indexing model from the issue text.