Mid-flight is all but certainly the best possible place for such an event to occur. It is interplanetary space. The debris cloud would disperse. As this is a constant-acceeleration design, an intercept orbit only exists during the initial and terminal phases of the nominal trajectory, such that debris would tend
not to intersect with either origin or destination planets for a considerable time, during which
both dispersion
and extensive radioactive decay would have occurred, both reducing net risks.
Planned mission profiles look to be Earth-Mars transfers. That's relatively nearby, but still a vast volume of space in which numerous other near-Earth-orbit objects exist posing relatively little risk. Including a Tesla automobile.
It would be a bad day for any crew and/or passengers, but would have minimal effects for anyone else, most likely.
Space is big. Even if we're only talking interplanetary space.