Error when passing shiny.tags to saveWidget()

[cpsievert]

When trying to save a collection of widgets (perhaps via crosstalk::bscols(), htmltools::tagList(), etc) this error message could be improved:

library(plotly)
library(leaflet)
library(crosstalk)
library(htmlwidgets)

tags <- tagList(plot_ly(), leaflet())
saveWidget(tags, "index.html")

Error in if (!(package %in% dev_packages())) { : 
  argument is of length zero

In fact, we could even make this work (and throw a warning if selfcontained = TRUE), as I've proposed in #309