Just to add more information - I also observed the same thing this morning. I've seen that this message appears relative to Rmd (html + shiny) apps and not the "ui.R + server.R" apps. Additionally, just as @pablobernabeu observed, this error occurs even if I just use the Rstudio default text for a Rmd (html + shiny document).
I put this document (the default from Rstudio) on my shinyapp.io with id: 1542533
Finally, this error also appears on my Department's (I'm a grad student) shiny server - so it probably isn't shinyapps.io specific.