Call to undefined method JApplicationSite::isClient()” when Updating to Joomla 3.8

When upgrading your Joomla based site to version 3.8 (or above) you can sometimes get the error: 'Call to undefined method JApplicationSite::isClient()'

This appears to have affected quite a few Joomla users.

In order to fix this, try taking the following steps:

1. Log into your hosting control panel and go to the file browser
2. Rename the folder in the root of your Joomla directory from 'libraries' to 'libraries_old'
3. Upload the 'libraries' folder from a fresh Joomla installation package
4. Test your site

This should resolve the issue temporarily. Now try updating all of your sites components, modules and plugins.

Test your site fully to make sure it is functioning as normal.