Comment 2 for bug 987410

Same issue, just upgraded to 12.04 from 11.10 amd64 arch.
The program 'filezilla' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
  (Details: serial 113860 error_code 3 request_code 152 minor_code 3)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

This appear on pressing OK or Cancel Button on Site Manager