From cb52851ed03159a7e31c66dbffe2ec3c7bb48523 Mon Sep 17 00:00:00 2001
From: Hossein Marzban <marzban98@gmail.com>
Date: Wed, 24 Oct 2018 15:59:10 +0330
Subject: [PATCH] fixe #2217, add error helper message

---
 js/reveal.js | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/js/reveal.js b/js/reveal.js
index 3c31b973774..e65d186fef6 100644
--- a/js/reveal.js
+++ b/js/reveal.js
@@ -553,6 +553,9 @@
 	 */
 	function setupDOM() {
 
+		if(dom.slides === null)
+			throw Error("RevealJs need a html element by class name of 'reveal' for initialize, like: <div class='reveal'></div>");
+
 		// Prevent transitions while we're loading
 		dom.slides.classList.add( 'no-transition' );