join := Parallel.Join(
procedure (const joinState: IOmniJoinState)
begin
...
end,
procedure (const joinState: IOmniJoinState)
begin
...
// don't use !! ERROR !! Application.MessageBox('Error', 'Error', 0);
MessageBox(0, 'Error', 'Error', 0); // That's the only way
...
end;
end).NoWait.Execute;