--- a/devtools/server/actors/layout.js
+++ b/devtools/server/actors/layout.js
@@ -120,21 +120,26 @@ var LayoutActor = ActorClassWithSpec(lay
    * iterating below the given rootNode and optionally including nested frames.
    *
    * @param  {NodeActor} rootNode
    * @param  {Boolean} traverseFrames
    *         Whether or not we should iterate through nested frames.
    * @return {Array} An array of GridActor objects.
    */
   getAllGrids: function (rootNode, traverseFrames) {
+    let grids = [];
+
+    if (!rootNode) {
+      return grids;
+    }
+
     if (!traverseFrames) {
       return this.getGrids(rootNode.rawNode);
     }
 
-    let grids = [];
     for (let {document} of this.tabActor.windows) {
       grids = [...grids, ...this.getGrids(document.documentElement)];
     }
 
     return grids;
   },
 
   onNavigate: function () {