[MultiStateSampler] Virtual sites cause NaNs if reassign_velocities=False

[IAlibay]

I'm trying to get some relative free energies done using tip4p/ew and I've noticed that I will get NaN's if reassign_velocities=False, but everything runs fine if I set it to True.

I need to dig into this a bit more to provide a reproducible example but I wanted to open this to check if someone might have any clues as to what's going on here? I know that there had been previous reports by the OpenFF folks about NaNs happening if setVelocitiesToTemperature & computeVirutalSites weren't called in the right order (see: openmm/openmm#3736), so I'm wondering if that's somehow related here?