This bugfix addresses a segfault in the "hatching" mode of the Calligraphic Tool (tracing an existing object, triggered with Ctrl key) when the reference path(s) have zero stroke width.
The key to preventing this crash is, unsurprisingly, more diligent null-pointer checking; some of the functions called in that part of the code are allowed to return nullptr by design, so the necessary checks have been introduced.
Fixes #3173 (closed)