title: Text to be written must be a length-one character vector
body: <p>After installing the new version of shiny (1.4.0) package, I face a strange error message </p>
  
  <pre><code>&gt; Warning: Error in writeImpl: Text to be written must be a length-one
  &gt; character vector   [No stack trace available]
  </code></pre>
  
  <p>I do not really understand what has been changed? And how to fix this issue?</p>
  
  <p>Any idea would be highly appreciated!</p>
  
  <p>The Trace looks as follows:</p>
  
  <pre><code>13: execCallbacks(timeoutSecs, all, loop$id)
  12: force(expr)
  11: with_loop(loop, invisible(execCallbacks(timeoutSecs, all, loop$id)))
  10: run_now(timeoutMs/1000, all = FALSE)
  9: service(timeout)
  8: serviceApp()
  7: ..stacktracefloor..(serviceApp())
  6: withCallingHandlers(expr, error = doCaptureStack)
  5: domain$wrapSync(expr)
  4: promises::with_promise_domain(createStackTracePromiseDomain(), 
         expr)
  3: captureStackTraces({
         while (!.globals$stopped) {
             ..stacktracefloor..(serviceApp())
         }
     })
  2: ..stacktraceoff..(captureStackTraces({
         while (!.globals$stopped) {
             ..stacktracefloor..(serviceApp())
         }
     }))
  1: runApp("Projects/Bomboleo")
  
  No traceback available 
  </code></pre>