When using this tool, sometimes the periapsis you end up with is not exactly the one you gave the tool, which is fine per se, like when returning from a moon, you just have to increase/decrease the specified value and recreate a maneuver node, and the resulting planned periapsis will be tweaked.
However, if you specify a negative altitude greater than the diameter of the body, the game crashes.

I understand it's an invalid value that can't be computed, but showing an error (like requesting an apoapsis lower than the altitude of the burn for example) would be better than letting the game crash.

The game also freezes if the requested periapsis is greater than the SOI.