When running Joomla on my local host or any other php environment that has the maximum or development error reporting enabled, the ajax request that the control panel received to determine whether there are updates available or not contains a php parser notice that prevents the JSON response to be interpreted correctly.

The error happens in Line 97 of the libraries/joomla/adapters/extension.php. It needs to be checked whether $this->currentUpdate is null before checking it's values in the if statement.