[devdocsgjs/main: 925/1867] Fix various favicon-related Sentry issues

[Andy Holmes <andyholmes src gnome org>]

commit 782429ca02ff13c2cdada9a6d02cff33530e81bf
Author: Jasper van Merle <jaspervmerle gmail com>
Date:   Thu Sep 5 14:21:23 2019 +0200

    Fix various favicon-related Sentry issues

 assets/javascripts/lib/favicon.coffee | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)
---
diff --git a/assets/javascripts/lib/favicon.coffee b/assets/javascripts/lib/favicon.coffee
index 8f3708e4..696bd784 100644
--- a/assets/javascripts/lib/favicon.coffee
+++ b/assets/javascripts/lib/favicon.coffee
@@ -28,12 +28,19 @@ withImage = (url, action) ->
     currentSlug = doc.slug
     return
 
-  styles = window.getComputedStyle($("._icon-#{doc.slug.split('~')[0]}"), ':before')
+  iconEl = $("._icon-#{doc.slug.split('~')[0]}")
+  return if iconEl == null
+
+  styles = window.getComputedStyle(iconEl, ':before')
+
+  backgroundPositionX = styles['background-position-x']
+  backgroundPositionY = styles['background-position-y']
+  return if backgroundPositionX == undefined || backgroundPositionY == undefined
 
   bgUrl = app.config.favicon_spritesheet
   sourceSize = 16
-  sourceX = Math.abs(parseInt(styles['background-position-x'].slice(0, -2)))
-  sourceY = Math.abs(parseInt(styles['background-position-y'].slice(0, -2)))
+  sourceX = Math.abs(parseInt(backgroundPositionX.slice(0, -2)))
+  sourceY = Math.abs(parseInt(backgroundPositionY.slice(0, -2)))
 
   withImage(bgUrl, (docImg) ->
     withImage(defaultUrl, (defaultImg) ->