When trying to create a PHP Project from an existing directory (File | New | PHP Project from Existing Directory) either nothing happens or an error "Argument cannot be null" is displayed.
Zend Studio 11
Use a longer path in the Document Root field of the default PHP Server's configuration:
The actual problem is that the Document Root of the defualt PHP Server's configuration is set to the root of the disk, e.g. C:\, D:\ (or / in UNIX-based systems).
The bug is fixed in Zend Studio 11.0.2.