R Shiny App Error - Couldn't Find Any UI

I wrote this code and initially Shiny worked just fine and the app started:

library(shiny)    
ui <- fluidPage(
  titlePanel("Title"),
  sidebarPanel(

    sliderInput("priceInput", "Price", min = 0, max = 100,
                value = c(25,40), pre = "$")
  ),
  mainPanel("Results")
)

But when I add any UI element to it, for example radioButton:

ui <- fluidPage(

  titlePanel("Title"),
  sidebarPanel(
    sliderInput("priceInput", "Price", min = 0, max = 100,
                value = c(25,40), pre = "$"),
    radioButtons("typeInput", "Product type",
                 choices = c("p1", "p2", "p3"),
                 selected = "p1")
  ),
  mainPanel("Results")
)

I get an error:

Shiny couldn't find any UI for this application. We looked in:    
www/index.html
ui.R
app.R

Any idea what might cause this? I tried other options instead of a radio button and received the same error. My server file is just an empty file with the server function.