gganimate not working with `geom_area()`

[AshesITR]

This looks related to #269, feel free to dupe-close if it's identical.

I'm trying to have a constant background reference plot using

• separate data and
• geom_area

This causes a crash related to -data$group.
Examining the stack trace suggests that the group aesthetic is assumed to be integer but something before the crashing code creates character groups instead.

data$group is constant and equal to "1" at the point of failure.
The failing function is positon_stack so geom_area is likely not the only affected geom.

library(gganimate)
library(dplyr)

p <- ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, group = Species, color = Species)) %+%
  geom_area(
    aes(group = NULL, color = NULL),
    data = iris %>%
      group_by(Sepal.Length) %>%
      summarise(Sepal.Width = mean(Sepal.Width)),
    alpha = 0.5
  ) %+%
  geom_point()

p

p %+%
  transition_states(Species) %>%
  animate(renderer = gifski_renderer(file = "temp.gif"))
#> Error in -data$group: invalid argument to unary operator

^{Created on 2020-10-29 by the reprex package (v0.3.0)}