Avoid many 'Failed to open directory' windows (Issue #660)

In this scenario, multiple 'Failed to open directory' error windows appear:
1- Navigate to a directory you can't usually access, such as '/root'
2- A 'Failed to open directory' error window appears. Don't close it
3- Copy some text to the clipboard from another application
4- Another 'Failed to open directory' error window appears
5- Repeat step 3 and 4 to open even more 'Failed to open directory' windows

Set the directory to NULL before instead of after opening the window to fix it