Bug 1052756 - Fix Toolbox.destroy() to properly clean up with add-on targets r=Mossop

browser/devtools/framework/toolbox.js

@@ -1358,8 +1358,10 @@ Toolbox.prototype = {
     // Destroying the walker and inspector fronts
     outstanding.push(this.destroyInspector().then(() => {
       // Removing buttons
-      this._pickerButton.removeEventListener("command", this._togglePicker, false);
-      this._pickerButton = null;
+      if (this._pickerButton) {
+        this._pickerButton.removeEventListener("command", this._togglePicker, false);
+        this._pickerButton = null;
+      }
     }));
 
     // Remove the host UI