Revert "widgetexplorer: Delete newStuffDialog on closed"
This reverts commit ee1cd1ab.
There will be a crash after the dialog is opened for the second time if an installation is in progress, because the dialog component is not deleted after QtQuickDialogWrapper is destructed.
The crash will be fixed by frameworks/knewstuff!179 (merged), so only revert the commit in 5.24 branch.