Ensure that we don't try and assign scale to NULL

Fixes #2599

Author: hadley

R/scale-type.R

@@ -1,4 +1,8 @@
-find_scale <- function(aes, x, env) {
+find_scale <- function(aes, x, env = parent.frame()) {
+  if (is.null(x)) {
+    return(NULL)
+  }
+
   type <- scale_type(x)
   candidates <- paste("scale", aes, type, sep = "_")
 

tests/testthat/test-scale-type.R

@@ -0,0 +1,12 @@
+context("test-scale-type.R")
+
+test_that("no scale for NULL aesthetic", {
+  expect_equal(find_scale("colour", NULL), NULL)
+})
+
+test_that("message + continuous for unknown type", {
+  x <- structure(1:10, class = "ggplot2_foo")
+
+  expect_message(scale <- find_scale("colour", x), "ggplot2_foo")
+  expect_s3_class(scale, "ScaleContinuous")
+})