You generally can't call methods on UI objects from any thread except the main thread (the one that starts when your application does). All of the UI objects live in this main thread. You can't create or modify UI objects from any other thread.
The only good way to update something in a UI object from a different thread is to make a signal / slot connection between the UI object and the worker thread. The signal will be handled properly across threads and the slot will be executed in the UI thread by the Qt event loop running in that thread, so no need for mutexes or other synchronization.
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