stage() should show a bit friendlier warning when used without after_stat and after_scale

[yutannihilation]

library(ggplot2)

ggplot(mpg, aes(class, hwy)) +
  geom_boxplot(aes(fill = stage(class)))
#> Error in uq_expr[[3]]: subscript out of bounds

^{Created on 2022-06-13 by the reprex package (v2.0.1)}