Python preprocessor freezes engine

Run the python preprocessing task in the background and somehow inform the user of the status. Right now, the preprocessing task freezes the editor for a long time and the user won't know if the engine crashed or is still calculating something. It even is unclear what would happen if the engine gets closed during the task. Will the subprocess continue running or stop as well? Is there any intermediate data that should be removed in case of an unexpected halt of the program?