fix: access nullptr when themeType changed

[18202781743]

SniTrayProtocolHandler may be destroyed ,but it's conection is
active.

[18202781743]

#0 0x00007f93802e6435 in DBusMenuImporter::updateMenu (this=0x0, force=true) at ./plugins/libdbusmenuqt/dbusmenuimporter.cpp:465
#1 0x00007f9387bec96c in QtPrivate::QSlotObjectBase::call (a=, r=, this=, this=, r=, a=)
at ./src/corelib/kernel/qobjectdefs_impl.h:486
#2 doActivate (sender=0x55a384343c90, signal_index=3, argv=0x7ffd87283fc0) at ./src/corelib/kernel/qobject.cpp:4120
#3 0x00007f9389174284 in Dtk::Gui::DGuiApplicationHelperPrivate::notifyAppThemeChanged() () from /lib/x86_64-linux-gnu/libdtk6gui.so.6
#4 0x00007f93891737f0 in Dtk::Gui::DGuiApplicationHelperPrivate::setPaletteType(Dtk::Gui::DGuiApplicationHelper::ColorType, bool) () from /lib/x86_64-linux-gnu/libdtk6gui.so.6
#5 0x00007f9389174019 in Dtk::Gui::DGuiApplicationHelper::setPaletteType(Dtk::Gui::DGuiApplicationHelper::ColorType) () from /lib/x86_64-linux-gnu/libdtk6gui.so.6
#6 0x00007f9387bec96c in QtPrivate::QSlotObjectBase::call (a=, r=, this=, this=, r=, a=)

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743, mhduiy

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[18202781743]

/forcemerge

[deepin-bot]

This pr force merged! (status: blocked)