Fix crash when @description or @details are NULL

Now they are ignored, except for `@description NULL` in
package level documentation, where it can be used to suppress the
auto-generated Description section

Closes #1008.

NEWS.md

@@ -3,6 +3,11 @@
 * roxygen2 now keeps using Windows (CR LF) line endings for files that
   already have CR LF line endings, and uses LF for new files (#989).
 
+* roxygen2 now does not fail for `@description NULL` and `@details NULL`.
+  Instead, these tags are ignored, except for `@description NULL` in
+  package level documentation, where it can be used to suppress the
+  auto-generated Description section (#1008).
+
 # roxygen2 7.0.2
 
 * `\example{}` escaping has been improved (again!) so that special escapes 

R/rd.R

@@ -191,7 +191,8 @@ topics_add_default_description <- function(topics) {
       next
 
     # rexport manually generates a own description, so don't need to
-    if (!topic$has_section("reexport")) {
+    if (!topic$has_section("reexport") &&
+        !identical(topic$get_value("docType"), "package")) {
       topic$add(rd_section("description", topic$get_value("title")))
     }
   }

R/topic.R

@@ -148,6 +148,7 @@ RoxyTopic <- R6::R6Class("RoxyTopic", public = list(
   #' @param section [rd_section] object to add.
 
   add_section = function(section, overwrite = FALSE) {
+    if (is.null(section)) return()
     type <- section$type
     if (self$has_section(type) && !overwrite) {
       section <- merge(self$get_section(type), section)

tests/testthat/test-rd.R

@@ -59,6 +59,85 @@ test_that("documenting NA gives useful error message (#194)", {
   )
 })
 
+test_that("@description NULL", {
+  # Just ignore in this case
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #'
+    #' @description NULL
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_identical(out[[1]]$get_value("description"), "Title")
+
+  # Still ignore
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #' @description NULL
+    #' @description desc
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_identical(out[[1]]$get_value("description"), "desc")
+
+  # Still ignore for objects as well
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #' @description NULL
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_identical(out[[1]]$get_value("description"), "Title")
+
+  # But drop for package docs
+  with_mock(
+    `roxygen2::read.description` = function(...)
+      list(Package = "roxygen_devtest",
+           Title = "Package Title",
+           Description = "Package description."),
+    out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+      #' Title
+      #'
+      #' @docType package
+      #' @description NULL
+      #' @name pkg
+      '_PACKAGE'
+    ")
+  )
+  expect_null(out[[1]]$get_value("description"))
+})
+
+test_that("@details NULL", {
+  # Just ignore in this case
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #'
+    #' @details NULL
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_null(out[[1]]$get_value("details"))
+
+  # Still ignore
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #' @details NULL
+    #' @details desc
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_identical(out[[1]]$get_value("details"), "desc")
+
+  # Still ignore for objects as well
+  out <- roxygen2::roc_proc_text(roxygen2::rd_roclet(), "
+    #' Title
+    #' @details NULL
+    #' @format NULL
+    foobar <- 1:10
+  ")
+  expect_null(out[[1]]$get_value("details"))
+})
+
 # UTF-8 -------------------------------------------------------------------
 
 test_that("can generate nonASCII document", {