Opening a file while a processing thread is active can clobber gfit and cause a crash
If some image processing is running in the processing thread and an image is opened, the processing thread will continue to run. When it completes, its output can clobber gfit and cause a crash.
Simply checking for the processing thread running and refusing to open the file doesn't work, because there appears to be a race condition with the file preview which runs in the thread, which for some users can cause Siril to refuse to open any files at all.