Comment 3 for bug 1585270

Based on tests with archived builds (on OS X 10.7.5):
- not reproduced with Inkscape 0.48.5 [1]
- not reproduced with lp:inkscape rev <= 13222 [1]
- reproduced with lp:inkscape rev >= 13224
the crash seems to have been exposed with the changes in
* Revision 13223: First step of refactoring SPKnot.
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/13223

[1] the earlier builds produce console messages when 'SHIFT'-dragging a vanishing point (to fork it), but do not crash when merging two vanishing points which had been separated earlier:

(inkscape-13222:56584): GLib-GObject-CRITICAL **: g_signal_handlers_disconnect_matched: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(inkscape-13222:56584): GLib-GObject-WARNING **: instance of invalid non-instantiatable type '(null)'