[R-package] Fix custom objective detection in `print.lgb.Booster()` (#4941)

* Update lgb.Booster.R

* Update lgb.cv.R

* Update lgb.train.R

* Update lgb.Booster.R

R-package/R/lgb.Booster.R

@@ -864,12 +864,13 @@ print.lgb.Booster <- function(x, ...) {
     if (obj == "none") {
       obj <- "custom"
     }
-    if (x$.__enclos_env__$private$num_class == 1L) {
+    num_class <- x$.__enclos_env__$private$num_class
+    if (num_class == 1L) {
       cat(sprintf("Objective: %s\n", obj))
     } else {
       cat(sprintf("Objective: %s (%d classes)\n"
           , obj
-          , x$.__enclos_env__$private$num_class))
+          , num_class))
     }
   } else {
     cat("(Booster handle is invalid)\n")

R-package/R/lgb.cv.R

@@ -141,7 +141,7 @@ lgb.cv <- function(params = list()
   fobj <- NULL
   if (is.function(params$objective)) {
     fobj <- params$objective
-    params$objective <- "NONE"
+    params$objective <- "none"
   }
 
   # If eval is a single function, store it as a 1-element list

R-package/R/lgb.train.R

@@ -109,7 +109,7 @@ lgb.train <- function(params = list(),
   fobj <- NULL
   if (is.function(params$objective)) {
     fobj <- params$objective
-    params$objective <- "NONE"
+    params$objective <- "none"
   }
 
   # If eval is a single function, store it as a 1-element list