This bug was reported on Ubuntu Launchpad: 
https://bugs.launchpad.net/bugs/448151

It has been duplicated by at least one other reporter.

From the launchpad report: 

after I closed this application this happened

ProblemType: Crash
Architecture: amd64
Date: Sat Oct 10 12:25:36 2009
DistroRelease: Ubuntu 9.10
ExecutablePath: /usr/bin/xfce4-appfinder
NonfreeKernelModules: nvidia
Package: xfce4-appfinder 4.6.1-1
ProcCmdline: xfce4-appfinder
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-13.43-generic
SegvAnalysis:
 Segfault happened at: 0x7fc2b6bf1b38: mov (%rcx),%rsi
 PC (0x7fc2b6bf1b38) ok
 source "(%rcx)" (0x00000000) not located in a known VMA region (needed readable region)!
 destination "%rsi" ok
SegvReason: reading NULL VMA
Signal: 11
SourcePackage: xfce4-appfinder
StacktraceTop:
 ?? () from /lib/libglib-2.0.so.0
 g_slice_free_chain_with_offset ()
 g_string_chunk_free () from /lib/libglib-2.0.so.0
 ?? () from /usr/lib/libthunar-vfs-1.so.2
 g_object_unref () from /usr/lib/libgobject-2.0.so.0
Title: xfce4-appfinder crashed with SIGSEGV in g_slice_free_chain_with_offset()
Uname: Linux 2.6.31-13-generic x86_64
UserGroups: adm admin cdrom dialout lpadmin plugdev sambashare

Plain text attachments are available as:

Stacktrace:
http://launchpadlibrarian.net/33703240/Stacktrace.txt

ThreadStacktrace:
http://launchpadlibrarian.net/33703251/ThreadStacktrace.txt

XsessionError log:
http://launchpadlibrarian.net/33414138/XsessionErrors.txt