Modernize code. Use nullptr where it's possible

Summary:
Remove extra ';'
Use override everywhere as we already use it in some part of code
Fix potential crash when we delete a pointer

Test Plan: build/using :)

Reviewers: hein

Reviewed By: hein

Subscribers: broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8473