--- a/devtools/client/inspector/boxmodel/box-model.js
+++ b/devtools/client/inspector/boxmodel/box-model.js
@@ -73,18 +73,18 @@ BoxModel.prototype = {
       onShowBoxModelHighlighter: this.onShowBoxModelHighlighter,
     };
   },
 
   /**
    * Returns true if the computed or layout panel is visible, and false otherwise.
    */
   isPanelVisible() {
-    return this.inspector.toolbox.currentToolId === "inspector" &&
-           this.inspector.sidebar &&
+    return this.inspector.toolbox && this.inspector.sidebar &&
+           this.inspector.toolbox.currentToolId === "inspector" &&
            (this.inspector.sidebar.getCurrentTabID() === "layoutview" ||
             this.inspector.sidebar.getCurrentTabID() === "computedview");
   },
 
   /**
    * Returns true if the layout panel is visible and the current node is valid to
    * be displayed in the view.
    */

--- a/devtools/client/inspector/grids/grid-inspector.js
+++ b/devtools/client/inspector/grids/grid-inspector.js
@@ -169,18 +169,18 @@ GridInspector.prototype = {
   getSwatchColorPickerTooltip() {
     return this.swatchColorPickerTooltip;
   },
 
   /**
    * Returns true if the layout panel is visible, and false otherwise.
    */
   isPanelVisible() {
-    return this.inspector.toolbox.currentToolId === "inspector" &&
-           this.inspector.sidebar &&
+    return this.inspector.toolbox && this.inspector.sidebar &&
+           this.inspector.toolbox.currentToolId === "inspector" &&
            this.inspector.sidebar.getCurrentTabID() === "layoutview";
   },
 
   /**
    * Load the grid highligher display settings into the store from the stored preferences.
    */
   loadHighlighterSettings() {
     let { dispatch } = this.store;