Fail on NULL engine (#1248)

* changes to #1242

* updated test cases

* Update R/engines.R

Co-authored-by: Simon P. Couch <[email protected]>

* rerun tests and redoc

* skip for upcoming dials release

---------

Co-authored-by: Simon P. Couch <[email protected]>

Author: topepo

NEWS.md

@@ -47,6 +47,8 @@
 
 * The quantile regression prediction type was disabled for the deprecated `surv_reg()` model. 
 
+* `NULL` is no longer accepted as an engine (#1242).
+
 
 # parsnip 1.2.1
 

R/engines.R

@@ -111,7 +111,7 @@ set_engine <- function(object, engine, ...) {
 set_engine.model_spec <- function(object, engine, ...) {
   mod_type <- class(object)[1]
 
-  if (rlang::is_missing(engine)) {
+  if (rlang::is_missing(engine) || is.null(engine)) {
     stop_missing_engine(mod_type, call = caller_env(0))
   }
   object$engine <- engine