Problem with GtkCellRendererCombo

[Rudolfo Pinewood <rudolfo pinewood googlemail com>]

Hello again,
I have a problem with GtkCellRendererCombo, for which I did not find a 
solution yet.

I try to use editing-started signal on that renderer to update combobox 
model based on current selection in my treeview. But it segfaults out 
when I try to receive the currently selected value from my treeview 
treemodel.

Segfault stacktrace: http://pastebin.com/m4694e64b

TreeView creation: http://pastebin.com/m4da78675

editing-started-callback: http://pastebin.com/mcdb04be
Note: here is the segfault in line 20 (gtk_tree_model_get), some things 
that drawed my attention: editable is 0x0, path is "0:0" (even if my 
selected line is the second one in my treeview)

Model update/creation: http://pastebin.com/m3f5f52b7

My questions:
- Does anybody find an error why it segfaults?
- Is my solution the prefered one to do such tasks? (I also tried to use 
cell_data function to update the combobox, this way was working to some 
extend, (combo got values, getting selection works) but seems to cause 
massive recalculating so it even does no longer support drag'n'drop in a 
second application part.)

Greetings,
Christoph Hartwig