executeElapsed gets stuck returning TRUE even when nothing was executed

Fixes #1278

R/timer.R

@@ -22,6 +22,11 @@ TimerCallbacks <- R6Class(
       .times <<- data.frame()
     },
     schedule = function(millis, func) {
+      # If args could fail to evaluate, let's make them do that before
+      # we change any state
+      force(millis)
+      force(func)
+
       id <- .nextId
       .nextId <<- .nextId + 1L
 
@@ -56,7 +61,7 @@ TimerCallbacks <- R6Class(
     },
     executeElapsed = function() {
       elapsed <- takeElapsed()
-      if (length(elapsed) == 0)
+      if (nrow(elapsed) == 0)
         return(FALSE)
 
       for (id in elapsed$id) {