--- a/devtools/client/responsive.html/browser/swap.js
+++ b/devtools/client/responsive.html/browser/swap.js
@@ -108,18 +108,20 @@ function swapToInnerBrowser({ tab, conta
       // Force the browser UI to match the new state of the tab and browser.
       thawNavigationState(tab);
       gBrowser.setTabTitle(tab);
       gBrowser.updateCurrentBrowser(true);
     }),
 
     stop() {
       // 1. Stop the tunnel between outer and inner browsers.
-      tunnel.stop();
-      tunnel = null;
+      if (tunnel) {
+        tunnel.stop();
+        tunnel = null;
+      }
 
       // 2. Create a temporary, hidden tab to hold the content.
       let contentTab = gBrowser.addTab("about:blank", {
         skipAnimation: true,
       });
       gBrowser.hideTab(contentTab);
       let contentBrowser = contentTab.linkedBrowser;
 

--- a/devtools/client/responsive.html/manager.js
+++ b/devtools/client/responsive.html/manager.js
@@ -373,39 +373,43 @@ ResponsiveUI.prototype = {
     // Ensure init has finished before starting destroy
     if (!isTabContentDestroying) {
       yield this.inited;
     }
 
     this.tab.removeEventListener("TabClose", this);
     this.tab.removeEventListener("BeforeTabRemotenessChange", this);
     this.browserWindow.removeEventListener("unload", this);
-    this.toolWindow.removeEventListener("message", this);
+    if (this.toolWindow) {
+      this.toolWindow.removeEventListener("message", this);
+    }
 
-    if (!isTabContentDestroying) {
+    if (!isTabContentDestroying && this.toolWindow) {
       // Notify the inner browser to stop the frame script
       yield message.request(this.toolWindow, "stop-frame-script");
     }
 
     // Destroy local state
     let swap = this.swap;
     this.browserWindow = null;
     this.tab = null;
     this.inited = null;
     this.toolWindow = null;
     this.swap = null;
 
     // Close the debugger client used to speak with emulation actor.
     // The actor handles clearing any overrides itself, so it's not necessary to clear
     // anything on shutdown client side.
-    let clientClosed = this.client.close();
-    if (!isTabContentDestroying) {
-      yield clientClosed;
+    if (this.client) {
+      let clientClosed = this.client.close();
+      if (!isTabContentDestroying) {
+        yield clientClosed;
+      }
+      this.client = this.emulationFront = null;
     }
-    this.client = this.emulationFront = null;
 
     if (!isWindowClosing) {
       // Undo the swap and return the content back to a normal tab
       swap.stop();
     }
 
     this.destroyed = true;