From befa9d130181ff99d155a6df454c40fba0d9736a Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Mon, 4 Jul 2022 14:24:57 +0200
Subject: [PATCH] Ensure that Nulls are ignored in creating slide shows.

Also ensure that Nulls are ignored in sectionification
by `makeSections`.

Closes #8155.
---
 src/Text/Pandoc/Shared.hs |  1 +
 src/Text/Pandoc/Slides.hs |  2 +-
 test/command/8155.md      | 19 +++++++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 test/command/8155.md

diff --git a/src/Text/Pandoc/Shared.hs b/src/Text/Pandoc/Shared.hs
index cc7893511..5c179158f 100644
--- a/src/Text/Pandoc/Shared.hs
+++ b/src/Text/Pandoc/Shared.hs
@@ -589,6 +589,7 @@ makeSections numbering mbBaseLevel bs =
     xs' <- go xs
     rest' <- go rest
     return $ Div attr xs' : rest'
+  go (Null:xs) = go xs
   go (x:xs) = (x :) <$> go xs
   go [] = return []
 
diff --git a/src/Text/Pandoc/Slides.hs b/src/Text/Pandoc/Slides.hs
index 2275bcffe..34763ea23 100644
--- a/src/Text/Pandoc/Slides.hs
+++ b/src/Text/Pandoc/Slides.hs
@@ -30,7 +30,7 @@ getSlideLevel = go 6
 
 -- | Prepare a block list to be passed to makeSections.
 prepSlides :: Int -> [Block] -> [Block]
-prepSlides slideLevel = ensureStartWithH . splitHrule . extractRefsHeader
+prepSlides slideLevel = ensureStartWithH . splitHrule . extractRefsHeader . filter (/= Null)
   where splitHrule (HorizontalRule : Header n attr xs : ys)
                        | n == slideLevel = Header slideLevel attr xs : splitHrule ys
         splitHrule (HorizontalRule : xs) = Header slideLevel nullAttr [Str "\0"] :
diff --git a/test/command/8155.md b/test/command/8155.md
new file mode 100644
index 000000000..966bff871
--- /dev/null
+++ b/test/command/8155.md
@@ -0,0 +1,19 @@
+```
+% pandoc -f native -t revealjs
+[ Null
+, Header
+    2
+    ( "header-after-null" , [] , [] )
+    [ Str "Header"
+    , Space
+    , Str "after"
+    , Space
+    , Str "null"
+    ]
+]
+^D
+<section id="header-after-null" class="title-slide slide level2">
+<h2>Header after null</h2>
+
+</section>
+```