ggplot2 does not currently support free scales with a non-cartesian coord or coord_flip

Question

With the results of LDA topic model, I am trying to create 30 horizontal bar charts to show top words vs their probabilities.

png("airport.png")
top_terms %>%
  mutate(term = reorder(term, beta)) %>%
  ggplot(aes(term, beta, fill = factor(topic))) +
  geom_bar(stat = "identity", show.legend = FALSE) +
  facet_wrap(~ topic, scales = "free") + 
  coord_flip()
dev.off()

The above codes returns an error:

Error in facet_render.wrap(plot$facet, panel, plot$coordinates, theme,  : 
  ggplot2 does not currently support free scales with a non-cartesian coord or coord_flip.

Any suggestions?

I also tried the following:

png("airport.png")
top_terms %>%
  mutate(term = reorder(term, beta)) %>%
  ggplot(aes(beta, term, fill = factor(topic))) +
  geom_bar(stat = "identity", show.legend = FALSE) +
  facet_wrap(~ topic, scales = "free")  
  #coord_flip()
dev.off()

The output looks like below. The problem is the graph does not show horizontal bars.