bs_themer() fixed to prevent NULL values from crashing themer, fix … (

#1112)

* `bs_themer()` fixed to prevent NULL values from crashing themer, fix #1111.

---------

Co-authored-by: Garrick Aden-Buie <garrick@adenbuie.com>

NEWS.md

@@ -2,6 +2,8 @@
 
 * `navset_card_pills()`, `navset_card_underline()`, `navset_card_tabs()` fixed to now respect header/footer arguments (@tanho63, #1024) 
 
+* Fixed a bug in `bs_themer()` (and `bs_theme_preview()`) that caused it to stop applying changes if a Sass variable was `NULL`. (@meztez, #1112)
+
 # bslib 0.8.0
 
 ## Breaking changes

R/bs-theme-preview.R

@@ -411,7 +411,7 @@ bs_themer <- function(gfonts = TRUE, gfonts_update = FALSE) {
     # Validate that `vals` is a simple list, containing atomic elements,
     # that are all named
     if (!identical(class(vals), "list") ||
-        !all(vapply(vals, is.atomic, logical(1))) ||
+        !all(vapply(vals, function(x) {is.atomic(x) || is.null(x)}, logical(1))) ||
         is.null(names(vals)) ||
         !isTRUE(all(nzchar(names(vals), keepNA = TRUE)))) {
       warning(call. = FALSE,