Steps to reproduce the issue

Install a file extension that includes a script file with in manifest xml. Scriptfile should have a postflight, preflight or unistall method with some custom code. Unistall the extension.
I necessary, I could create a very small test installation zip for you.

Expected result

preflight, uninstall and postflight methods in manifest script (script.php) are executed

Actual result

Non of the methods is executed but script file is deleted properly

System information (as much as possible)

Joomla! 4.0.0-dev Development [ Amani ] 19-November-2017 16:34 GMT
Local XAMPP Apache/2.4.26 (Win32) OpenSSL/1.0.2l PHP/7.1.7 on Windows 7

Additional comments

This ussue only happens on uninstall. Install works fine.
I think basically the problem is, that the getElement() function in the FileAdapter class tries to locate the script file with
$manifestPath = \JPath::clean($this->parent->getPath('manifest'));
but this path is not set at any time for the FileAdapter
On other adpaters (like PackageAdapter, PluginAdapter this issue does not happen, too).