title: R Shiny Crashing for invalid user input (Error argument of length 0)
body: <p>I'm working on an R Shiny app that that allows users to enter their own formulas for a given model. When users enter an invalid input (say two plus signs in a row), the app crashes with the following error:</p>
  <p><code>Warning: Error in :: argument of length 0</code></p>
  <p>I'm having trouble controlling user input and locating where exactly the error arises. The object is a data frame/list. I've tried to use if loops with <code>is.na</code> and <code>is.null</code> as a condition but I still get this error. How could one control for this error in Shiny, say for any invalid input that the user could enter?</p>
  <p>Thank you!</p>