So the only problem can be if the visualizer tries to access exactly the same index in the QList that is written by the loader.
Right.
And I am not sure if this behavior is defined.
So if during the writing of the list, it gets disrupted because of the access on it from your gui, resulting in messedup content of the list element, your messedup element stays there since you are only filling it ones.
It vary bad practice not to mutex shared data between threads, even if you think that your case does not need it.
I guess the easiest thing would be to try it, its just few lines of code.

Having said that, you didn't show your code, so it is very possible you are doing something wrong somewhere completely else.