CSS: window.getDefaultComputedStyle may return null

Fixes #14736
Closes gh-1501
(cherry picked from commit 51910ba8a5aff4cd1a811f1fdb76a6c62a09a666)

AUTHORS.txt

@@ -218,4 +218,5 @@ Forbes Lindesay <forbes@lindesay.co.uk>
 S. Andrew Sheppard <andrew@wq.io>
 Roman Reiß <me@silverwind.io>
 Benjy Cui <benjytrys@gmail.com>
+Rodrigo Rosenfeld Rosas <rr.rosas@gmail.com>
 

src/css/defaultDisplay.js

@@ -13,14 +13,15 @@ var iframe,
  */
 // Called only from within defaultDisplay
 function actualDisplay( name, doc ) {
-	var elem = jQuery( doc.createElement( name ) ).appendTo( doc.body ),
+	var style,
+		elem = jQuery( doc.createElement( name ) ).appendTo( doc.body ),
 
 		// getDefaultComputedStyle might be reliably used only on attached element
-		display = window.getDefaultComputedStyle ?
+		display = window.getDefaultComputedStyle && ( style = window.getDefaultComputedStyle( elem[ 0 ] ) ) ?
 
 			// Use of this method is a temporary fix (more like optmization) until something better comes along,
 			// since it was removed from specification and supported only in FF
-			window.getDefaultComputedStyle( elem[ 0 ] ).display : jQuery.css( elem[ 0 ], "display" );
+			style.display : jQuery.css( elem[ 0 ], "display" );
 
 	// We don't have any data stored on the element,
 	// so use "detach" method as fast way to get rid of the element