`method_` doesn't error correctly if method not found

[hadley]

library(S7)

foo <- new_generic("foo", "x")
method(foo, object = "x")
#> Error in method(foo, object = "x"): 'Rf_getCharCE' must be called on a CHARSXP, but got 'NULL'

^{Created on 2023-11-27 with reprex v2.0.2.9000}