Easy solution = press stop, save project, add vst (or vsti), hope cubase doesnt crash.
Proper solution = steinberg fixes issue allowing any vst to be added at any point in the track while playing without cubase crashing
1 Play project with 1-4 instruments and midi data.
2 Add vst effect to any of the instruments
3 “cubase has stopped working”
(frequency of bug estimated at 1 in 6 times)