crash after using Edit nodes icon twice (rev >= 12730)

Bug Description

Windows XP, Inkscape rev 12792

- draw a Bezier curve (shift F6)
- hit Enter to terminate drawing of Bezier
- click on Edit nodes (F2)
- click on one of the endpoints of the Bezier
- click on Edit nodes again
- crash

backtrace:

C:\InkscapeBZR\inkscape>gdb inkscape
GNU gdb 6.8
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(no debugging symbols found)
(gdb) symbol-file inkscape.dbg
Reading symbols from C:\InkscapeBZR\inkscape/inkscape.dbg...done.
(gdb) run
Starting program: C:\InkscapeBZR\inkscape/inkscape.exe
[New thread 3868.0xf24]
[New thread 3868.0x820]
[New thread 3868.0x578]

Program received signal SIGSEGV, Segmentation fault.
0x00d5d83e in sp_node_toolbox_coord_changed ()
(gdb) bt
#0 0x00d5d83e in sp_node_toolbox_coord_changed ()
#1 0x00000f11 in ?? ()
#2 0x00000001 in ?? ()
#3 0x077f4850 in ?? ()
#4 0x0000000b in ?? ()
#5 0x077f4850 in ?? ()
#6 0x058bd4b0 in ?? ()
#7 0x00b5ad8a in Inkscape::UI::Dialogs::SwatchesPanel::_updateFromSelection
()
#8 0x07809280 in ?? ()
#9 0x03e36c00 in ?? ()
#10 0x7c91005d in ntdll!RtlFreeThreadActivationContextStack ()
from C:\WINDOWS\system32\ntdll.dll
#11 0x00000000 in ?? ()

Reproduced on Crunchbang Waldorf, Inkscape trunk revision 12841.
Not reproduced with Inkscape 0.48.3.1.

Testing with archived builds:
- not reproduced with rev <= 12729
- reproduced with rev >= 12733

Possibly related to changes in 12730?
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/12733>

> - click on one of the endpoints of the Bezier

AFAICT not specific to endpoints - happens whenever the node tool is re-activated with a current selection of nodes (also triggered by typing the keyboard shortcut 'N' while having a random amount of middle nodes selected).

Confirmed on vista64 and fixed in trunk with commit r12903.
Seems weird though that _selected_nodes can be NULL...