Unable to test `renderDT()` in a `MockShinySession`

[cpsievert]

The following code generates an error when I'd expect it to print the JSON sent to the browser.

library(shiny)
library(DT)

session <- MockShinySession$new()
isolate({
  outFunc <- renderDT({
    head(mtcars)
  })
  outFunc(session, "name")
})

Error in shiny::snapshotPreprocessInput(paste0(name, "_state"), function(value) { : 
  snapshotPreprocessInput() needs a session object.