When an application is about to be closed and all the destructors are called, if there is QtConcurrent::run on the way, it crashes as the internal threadpool pointer is nullptr.

To reproduce:
1) Start the application.
2) Close the application
-> The program has unexpectedly finished.

The crash happens in mainwindow.cpp -> QtConcurrent::run.

Form the code it is not always clear what is the reason for running a thread, so such crashes can only be avoided by this check:

    QThreadPool* threadPool = QThreadPool::globalInstance();
    if(threadPool)
        QtConcurrent::run(..);