Protects from killing a closed process

test/webbrowser.js

@@ -99,8 +99,10 @@ WebBrowser.prototype = {
       this.callback = callback;
     }
 
+    if (this.process) {
       this.process.kill('SIGTERM');
     }
+  }
 };
 
 var firefoxResourceDir = path.join(__dirname, 'resources', 'firefox');