[Background Finder] Create a valid invocation token

Summary: According to Qt docs, QUuid() creates a null UUID object.

Test Plan: N/A, plasmashell is very unstable with Qt 5.15.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27237

wallpapers/image/backgroundlistmodel.cpp

@@ -491,7 +491,7 @@ const QStringList BackgroundListModel::wallpapersAwaitingDeletion()
 BackgroundFinder::BackgroundFinder(Image *wallpaper, const QStringList &paths)
     : QThread(wallpaper),
       m_paths(paths),
-      m_token(QUuid().toString())
+      m_token(QUuid::createUuid().toString())
 {
 }