--- a/devtools/server/actors/layout.js
+++ b/devtools/server/actors/layout.js
@@ -93,16 +93,20 @@ var LayoutActor = ActorClassWithSpec(lay
    *
    * @param  {Node|NodeActor} rootNode
    *         The root node to start iterating at.
    * @return {Array} An array of GridActor objects.
    */
   getGrids: function (rootNode) {
     let grids = [];
 
+    if (!rootNode) {
+      return grids;
+    }
+
     let treeWalker = this.walker.getDocumentWalker(rootNode);
     while (treeWalker.nextNode()) {
       let currentNode = treeWalker.currentNode;
 
       if (currentNode.getGridFragments && currentNode.getGridFragments().length > 0) {
         let gridActor = new GridActor(this, currentNode);
         grids.push(gridActor);
       }