Klipper: Avoid a crash on exit with the legacy application

Caused by the context menu and the associated widget being set to the
same object, leading to an access after destruction.

 Application: Klipper (klipper), signal: Segmentation fault
 Content of s_kcrashErrorMessage: std::unique_ptr<char []> = {get() = 0x0}
 #8 QObject::disconnect (member=0x0, ...)
 #9 KStatusNotifierItem::~KStatusNotifierItem () at knotifications/src/kstatusnotifieritem.cpp:71
 #10 KlipperTray::~KlipperTray () at plasma-workspace/klipper/tray.h:13