fix #1487: better error handling for insertUI when selector does not match anything in DOM

[bborgesr]

This makes the JS error more descriptive. (Still no error is printed to the R console or any indication appears on app that something went wrong).

Example app (notice that I use the wrong selector #add1, instead of #add):

library(shiny)
options(shiny.launch.browser=F, shiny.minified=F, shiny.port = 9000)

## Only run this example in interactive R sessions
if (interactive()) {
  # Define UI
  ui <- fluidPage(
    actionButton("add", "Add UI")
  )
  
  # Server logic
  server <- function(input, output, session) {
    observeEvent(input$add, {
      insertUI(
        selector = "#add1",
        where = "afterEnd",
        ui = textInput(paste0("txt", input$add),
                       "Insert some text")
      )
    })
  }
  
  # Complete app with UI and server components
  shinyApp(ui, server)
}

cc @wch

[wch]

If I understand correctly, if targets.length is 0, it will always reach this throw statement. This is because in line 192 of output_binding_html.js, it calls val.replace(self._reSingleton, findNewPayload), but val is [], so there's no replace method.

If I'm right about that, then this code path (where targets.length===0) never worked, and that should be fixed. That should probably be done first. When would it be desirable to render this stuff when there are no targets?

[jcheng5]

Yeah it looks like this call has transposed arguments, should be:

exports.renderHtml(message.content.html, $([]), message.content.deps)

[jcheng5]

I don't think we should throw, rather just log a warning. The dispatchMessage mechanism isn't designed for message handlers to throw exceptions (though maybe/probably it should be) and anyway this isn't that crazy of a thing to do, especially with multiple = TRUE.

[bborgesr]

@jcheng5, just console.log, then?