gganimate fails with start_cap/end_cap in ggraph

[mjsmith037]

While gganimate behaves as expected for simple ggraphs, when adding edge effects, such as start_cap or end_cap, it fails:

library(tidyverse)
library(tidygraph)
library(ggraph)
library(gganimate)
library(lubridate)

net_data <- play_erdos_renyi(10, 0.5) %>%
  activate(edges) %>%
  mutate(onset=as_datetime(1:n()), terminus=onset + 1)

ggraph(net_data) + geom_node_point() + geom_edge_link() +
  transition_events(start=onset, end=terminus, enter_length=origin, exit_length=origin)
#> Using `stress` as default layout

ggraph(net_data) + geom_node_point() + geom_edge_link(start_cap=circle())
#> Using `stress` as default layout

ggraph(net_data) + geom_node_point() + geom_edge_link(start_cap=circle()) +
  transition_events(start=onset, end=terminus, enter_length=origin, exit_length=origin)
#> Using `stress` as default layout
#> Warning: Cannot get dimensions of plot table. Plot region might not be
#> fixed
#> Warning: x must be a geometry object
#> ...
#> Warning: x must be a geometry object
#> Error in png::readPNG(frames[1], native = TRUE): unable to open /tmp/RtmpYdHlwj/28ca6bf2041c/gganim_plot0001.png

^{Created on 2019-09-25 by the reprex package (v0.3.0)}