When opening an item in the Workbox the content tree is disabled by default starting from Sitecore 8.2.3 release. The following article describes how to get the designed behavior set to a default value.
To change the default behavior, consider one of the following options:
if (command.value.indexOf("Content-Editor") != -1){ command.value = command.value.split('&mo=popup').join('');
command.value = command.value.split('mo=popup&').join('');
}