But I don't use Qt in my program in any ways.
I think the LabView GUI is Qt-based. However, if you say that when your developer modified the code, the crash went away, it sounds to me like the problem is the relationship between the developer's threading code and the LabView internals. Likely there was either a timing issue over resource cleanup or the threading code was causing memory corruption that was not revealed until the app went through its exit shutdown.