fix: lastFocusedInputContext may be nullptr.

[Xinkai]

Stack trace of thread 729795:
#0  0x00007fcd4ec6cf64 fcitx::InputContext::uuid() const (libFcitx5Core.so.7 + 0x6ef64)
#1  0x00007fcd4c42fda3 DictypeFcitx::trigger_(fcitx::KeyEvent const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const::$_2::operator()(DictypeFcitx const*) const (libdictype.so + 0x19da3)
#2  0x00007fcd4eb6928b n/a (libFcitx5Utils.so.2 + 0x4b28b)
#3  0x00007fcd4eb60e26 n/a (libFcitx5Utils.so.2 + 0x42e26)
#4  0x00007fcd4eb65c0c n/a (libFcitx5Utils.so.2 + 0x47c0c)
#5  0x00007fcd4e3d5163 n/a (libsystemd.so.0 + 0x41163)
#6  0x00007fcd4e3da4f3 sd_event_dispatch (libsystemd.so.0 + 0x464f3)
#7  0x00007fcd4e3da769 sd_event_run (libsystemd.so.0 + 0x46769)
#8  0x00007fcd4e3da950 sd_event_loop (libsystemd.so.0 + 0x46950)
#9  0x00007fcd4eb9474c fcitx::EventLoop::exec() (libFcitx5Utils.so.2 + 0x7674c)
#10 0x00007fcd4ec5099e fcitx::Instance::exec() (libFcitx5Core.so.7 + 0x5299e)
#11 0x0000560006f1ac8e n/a (/usr/bin/fcitx5 + 0x12c8e)
#12 0x00007fcd4e4eb6c1 n/a (libc.so.6 + 0x276c1)
#13 0x00007fcd4e4eb7f9 __libc_start_main (libc.so.6 + 0x277f9)
#14 0x0000560006f1c095 n/a (/usr/bin/fcitx5 + 0x14095)