Fix spectacle crash

focusWindow may have no platform window.

#0  0x00007f83690a7a2c in ??? () at /usr/lib/libc.so.6
#1  0x00007f836904d1a0 in raise () at /usr/lib/libc.so.6
#2  0x00007f8370f1957c in KCrash::defaultCrashHandler (sig=11) at /data/projects/kde/src/kcrash/src/kcrash.cpp:623
#3  0x00007f836904d2d0 in <signal handler called> () at /usr/lib/libc.so.6
#4  0x00007f837122a67e in QtWaylandClient::QWaylandInputDevice::serial (this=0x1b480)
    at /data/projects/kde/src/qt6/qtbase/src/plugins/platforms/wayland/qwaylandinputdevice_p.h:227
#5  0x00007f8371227c8e in LayerShellQt::QWaylandLayerSurface::requestActivate (this=0x5627ad75cae0)
    at /data/projects/kde/src/layer-shell-qt/src/qwaylandlayersurface.cpp:209
#6  0x00007f836e37ce10 in QtWaylandClient::QWaylandWindow::requestActivateWindow (this=<optimized out>)
    at /data/projects/kde/src/qt6/qtbase/src/plugins/platforms/wayland/qwaylandwindow.cpp:1596
#7  0x00007f836a0312ac in QWindow::requestActivate (this=this@entry=0x5627ad57acb0)
    at /data/projects/kde/src/qt6/qtbase/src/gui/kernel/qwindow.cpp:1303
#8  0x00007f836a0313a3 in QWindow::showFullScreen (this=0x5627ad57acb0)
    at /data/projects/kde/src/qt6/qtbase/src/gui/kernel/qwindow.cpp:2403
#9  0x00007f836a03217e in QWindow::setVisibility (this=<optimized out>, v=<optimized out>)
    at /data/projects/kde/src/qt6/qtbase/src/gui/kernel/qwindow.cpp:340
#10 0x000056279b3852ac in SpectacleWindow::setVisibilityForAll (visibility=QWindow::FullScreen)
    at /data/projects/kde/src/spectacle/src/Gui/SpectacleWindow.cpp:109
#11 0x000056279b3e00d9 in operator() (__closure=0x5627abad7470, rect=..., actions=...)
    at /data/projects/kde/src/spectacle/src/SpectacleCore.cpp:156
#12 0x000056279b3f472c in operator() (__closure=0x7ffef33be580)