'Sort Attributes': crash with extensions live preview if <style> element in drawing

[su_v]

The recently added feature to sort attributes of SVG elements may trigger a crash when toggling extensions' Live preview off if the document contains a <style> element (inline stylesheet).

Steps to reproduce:
0) edit Inkscape's preference to enable the feature
   (see bug #1045464 comment 3 for details)
1) launch inkscape
2) open attached test case
   (default document, with empty inline stylesheet added)
3) open 'Extensions > Render > Gear > Gear ...'
4) toggle Live preview on and off again

--> inkscape crashes when toggling off.

Notes:
- The crash does not occur when applying the extension (it seems limited to 'Live preview' and the involved undo to restore the original file when turning live preview off).
- The crash does not occur if sort_attributes is disabled ('0'), nor does it happen after deleting the inline stylesheet before opening the document.
- Some details in backtraces may vary depending on whether the stylesheet has content or not, but AFAICT that does not prevent or otherwise affect the observed crashes.

Reproduced with lp:inkscape/0.92.x r15317 on OS X 10.7.5.

[su_v (suv-lp)]

Backtrace with empty stylesheet, using lp:inkscape/0.92.x r15317