hejie615 you're quite right, I've just managed to reproduce this error when printing a library symbol with at least one override point via
The problem seems be to that
Override.selected property getter doesn't quite work well for library symbols and
console.log does query this property when printing out a symbol.
I've reported this bug to Sketch, but in the meantime the only workaround I can think of is to use
print() instead of
console.log for printing out library symbols and to avoid using
Override.selected property on them.