fix: do not error when converting datatype Null to R

NEWS.md

@@ -2,9 +2,14 @@
 
 ## Polars R Package (development version)
 
+### Bug fixes
+
+- Converting data of datatype `Null` to R doesn't error anymore. It now creates
+  a column filled with `NA` (#1217).
+
 ## Polars R Package 0.19.1
 
-- This is a maintenance relase. No user facing changes.
+- This is a maintenance release. No user facing changes.
 
 ## Polars R Package 0.19.0
 

R/dataframe__frame.R

@@ -992,9 +992,19 @@ DataFrame_group_by = function(..., maintain_order = polars_options()$maintain_or
 #' df$to_data_frame()
 DataFrame_to_data_frame = function(..., int64_conversion = polars_options()$int64_conversion) {
   # do not unnest structs and mark with I to also preserve categoricals as is
-  l = lapply(self$to_list(unnest_structs = FALSE, int64_conversion = int64_conversion), I)
+  l = lapply(
+    self$to_list(unnest_structs = FALSE, int64_conversion = int64_conversion),
+    function(x) {
+      # correctly handle columns with datatype Null
+      if (is.null(x)) {
+        NA
+      } else {
+        I(x)
+      }
+    }
+  )
 
-  # similar to as.data.frame, but avoid checks, whcih would edit structs
+  # similar to as.data.frame, but avoid checks, which would edit structs
   df = data.frame(seq_along(l[[1L]]), ...)
   for (i in seq_along(l)) df[[i]] = l[[i]]
   names(df) = .pr$DataFrame$columns(self)

tests/testthat/test-dataframe.R

@@ -579,6 +579,23 @@ test_that("to_Struct, unnest, to_frame, to_data_frame", {
   expect_identical(df$to_data_frame(), df_e)
 })
 
+test_that("conversion of datatype Null to R works", {
+  df = pl$DataFrame(x = NULL)
+  expect_identical(
+    df$to_data_frame(),
+    data.frame(x = NA)
+  )
+
+  df2 = pl$DataFrame(x = 1:2, y = NULL)
+  expect_identical(
+    df2$to_data_frame(),
+    data.frame(x = 1:2, y = c(NA, NA))
+  )
+  expect_identical(
+    as.data.frame(df2),
+    data.frame(x = 1:2, y = c(NA, NA))
+  )
+})
 
 test_that("unnest works correctly", {
   df = pl$DataFrame(