Fix update of the preset editor GUI when a default preset is created

Summary:
Before the patch, preset editor's GUI was set to the previous
paintop type, which was incorrect. I'm not entirely sure that
this fix is correct, so putting it for review.

CCBUG:392869

Test Plan:
1) Press "New default preset"
2) See that the GUI updated correctly

Questions:
1) Is it really default preset got loaded? Or something else?

Reviewers: #krita, scottpetrovic

Differential Revision: https://phabricator.kde.org/D12065