to avoid the crash during VST scan you can revert to the infamous dummy-VST scan... that way it won't actually load any VST(.so) plugins during scan but it will crash if you insist to select and open any that is based on qt4 though;
one side-effect of the so called "dummy-VST scan" is that it won't show any VST-shell sub-plugins for selection thereafter; however you can still work with any previously saved sessions that include any of those.
also note that eventually, if you still run older qtractor-qt4 in presence of newer qt5-plugins, the same crash-issue applies as in a vice-versa scenario.
to avoid the crash during VST scan you can revert to the infamous dummy-VST scan... that way it won't actually load any VST(.so) plugins during scan but it will crash if you insist to select and open any that is based on qt4 though;
one side-effect of the so called "dummy-VST scan" is that it won't show any VST-shell sub-plugins for selection thereafter; however you can still work with any previously saved sessions that include any of those.
also note that eventually, if you still run older qtractor-qt4 in presence of newer qt5-plugins, the same crash-issue applies as in a vice-versa scenario.
hth.
cheers