ensure_one(): reduce noise if attrs are NULL

ignore NULL values

R/subplots.R

@@ -339,14 +339,18 @@ dots2plots <- function(...) {
 # helper function that warns if more than one plot-level attribute 
 # has been specified in a list of plots (and returning that attribute)
 ensure_one <- function(plots, attr) {
-  attrs <- lapply(plots, "[", attr)
+  attrs <- Filter(Negate(is.null), lapply(plots, "[[", attr))
+  if (length(attrs) == 0) {
+    warning("No ", attr, " found", call. = FALSE)
+    return (NULL)
+  }
   for (i in seq_along(attrs)) {
     if (!identical(attrs[[1]], attrs[[i]])) {
       warning("Can only have one: ", attr, call. = FALSE)
       break
     }
   }
-  attrs[[length(attrs)]][[1]]
+  attrs[[length(attrs)]]
 }