It's not the reason of ths bug.
IPersistStreamInit is in an Activex which can't be threaded ;(
I've changed function TWebGMaps.Launch: Boolean;
I've replace (FWebBrowser.Document as IPersistStreamInit).Load(TStreamAdapter.Create(aStream));
with the following lines
lst:= TStringList.Create;
lst.Text := HtmlFile;
lst.SaveToFile('o:\map.html');
FWebBrowser.Navigate('o:\map.html');
lst.free;
So Now I can use a TThread to launch the map.
I don't like this version because I must write on the hard drive, but I don't see how to do otherwise.
Maybe you'll have a better idea