Bug 1556135 - only check if the current node is null in getClosestBackgroundImage. r=gl

Differential Revision: https://phabricator.services.mozilla.com/D33396

--HG--
extra : moz-landing-system : lando

devtools/server/actors/inspector/utils.js

@@ -302,7 +302,7 @@ function getClosestBackgroundColor(node) {
 function getClosestBackgroundImage(node) {
   let current = node;
 
-  while (current.ownerDocument) {
+  while (current) {
     const computedStyle = CssLogic.getComputedStyle(current);
     if (computedStyle) {
       const currentBackgroundImage = computedStyle.getPropertyValue("background-image");