Plasma 5.14.4 crash fix

[Jonathan Riddell]

Problem breaking taskbar window grouping and occasional crash in Plasma 5.14.4
https://bugs.kde.org/show_bug.cgi?id=401508

Please add this fix
https://cgit.kde.org/plasma-desktop.git/commit/?h=Plasma/5.14&id=ed34cc5f181e61d1fc98872866c5d7300a90af86

(I can make a fresh tar if anyone thinks that's useful but it seems
just as easy to add the patch)

Jonathan Riddell