I get a pop up and a crash error report when I switch while my current character is flying with the Jetpack.

The mod Bob's Classes and Multiple characters mod (3.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event bobclasses::on_gui_click (ID 1)
LuaEntity::active is read only.
stack traceback:
[C]: in function 'newindex'
__bobclasses/control.lua:1524: in function 'switch_character'
bobclasses/control.lua:721: in function <bobclasses/control.lua:696>

That's an issue with the jetpack mod. It was reported to them a long while back.

Ok, fair enough.
Curious why it's showing as an issue in this mod though (from the technical side)?

Fixed in v3.0.2