Critical message when live-previewing an extension in document with filter effects

Live-previewing an effect extension in a document which has objects referencing a filter effect produces these warnings (one per filtered objects) when toggling off the live preview:

** (inkscape:68369): CRITICAL **: void SPObject::requestModified(unsigned int): assertion 'this->document != NULL' failed

Steps to reproduce:
1) launch inkscape (default prefs, default new doc)
2) draw a shape
3) apply a filter effect
4) open 'Extensions > Render > Gear > Gear'
5) toggle 'Live preview' on and off again

-> one critical warning on the console.

Based on tests with archived builds (on OS X 10.7.5):
- not reproduced with Inkscape 0.91 r13725
- not reproduced with lp:inkscape rev <= 13822,
- reproduced with lp:inkscape rev >= 13824,
- reproduced with lp:inkscape/0.92.x r15301;
these warnings seem to have been exposed with the changes in r13824:
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/13824

Rev 13824: Replace sp_style_xxx functions with SPStyle member functions.
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/13824

Reproduced with Inkscape 0.92pre5 r15287 on Windows 10 and Inkscape 0.92.0 r15299 on OS X 10.7.5.