Fixes the following backtrace:
*0  KWaylandServer::DataDeviceInterface::dragSource() const (this=0x0)
at ./src/server/datadevice_interface.cpp:199
*1  0x00007f10d67b0c71 in
KWaylandServer::DataDeviceInterface::updateDragTarget(KWaylandServer::SurfaceInterface*,
unsigned int) (this=0x55c42e3ee9a0,
surface=surface@entry=0x55c42e4b3170, serial=serial@entry=3104)
    at ./src/server/datadevice_interface.cpp:278
*2  0x00007f10d67d8e52 in
KWaylandServer::SeatInterface::setDragTarget(KWaylandServer::SurfaceInterface*,
QPointF const&, QMatrix4x4 const&)
(this=this@entry=0x55c42d422ed0, surface=0x55c42e4b3170,
globalPosition=..., inputTransformation=...) at
/usr/include/c++/9/bits/atomic_base.h:413
*3  0x00007f10d67d9209 in
KWaylandServer::SeatInterface::setDragTarget(KWaylandServer::SurfaceInterface*,
QMatrix4x4 const&) (this=this@entry=0x55c42d422ed0, surface=<optimized
out>, inputTransformation=...)
    at ./src/server/seat_interface.cpp:578
*4  0x000055c42cb4563a in KWin::Xwl::XToWlDrag::setDragTarget()
(this=this@entry=0x55c42ea07a00) at ./src/toplevel.h:990
*5  0x000055c42cb47a68 in KWin::Xwl::XToWlDrag::offerCallback(QString
const&) (mime=..., this=0x55c42ea07a00) at ./src/xwl/drag_x.cpp:242
*6  KWin::Xwl::XToWlDrag::offerCallback(QString const&)
(this=0x55c42ea07a00, mime=...) at ./src/xwl/drag_x.cpp:237
*7  0x00007f10d5dc06fe in  () at /lib/x86_64-linux-gnu/libQt5Core.so.5