Qt Bridge export hangs if a file with the same name as export directory exists.

The Extendscript API fails to differentiate between the files and the directories. This causes the export workflow to assume that the export directory is already created whereas the directory is a file and the generator plugin fails to write the assets at the export path.

The Extenscript 'Folder' class behaves unexpectedly here and also the generator. The asset generator promise should call reject instead it silently failing.