If you unlink the vertical and horizontal radius and set the horizontal one to zero while the vertical one is higher than zero, applying the filter or having the preview mode on causes Krita to shut down. Strangely enough, setting the vertical radius to zero and the horizontal radius to a value above zero does not create the same effect. I have not tested this on gaussian blur or any other filters that also might have adjustable radius values.
Created attachment 110050 [details] (GDB Report) I can reproduce the crash on Ubuntu 14.04 - Krita 4.0.0-beta1.1.appimage. Steps: 1) "Filter > Blur > Blur..." 2) unlink Horizontal and Vertical radius 2) set the Horizontal radius = 0 (Vertical radius has to be > 0)
(Confirming)
Git commit eea93b7af358c0c73d1ff291f23099c1a745bd00 by Boudewijn Rempt. Committed on 22/01/2018 at 14:29. Pushed by rempt into branch 'master'. Set the minimum blur radius to 1 M +12 -6 plugins/filters/blur/wdgblur.ui https://commits.kde.org/krita/eea93b7af358c0c73d1ff291f23099c1a745bd00