Keyboard destroyed values unset

Fix keyboard errors because trying to reuse invalid properties.

The errors I'm fixing are the one mentioned in issue #1281 (closed) and this trace (when toggling OSK various times)

(gnome-shell:11042): GLib-CRITICAL **: 15:04:17.447: Source ID 4319 was not found when attempting to remove it
== Stack trace for context 0x55f801412260 ==
#0   55f801681a00 i   /media/M2/GNOME/gnome-shell/js/ui/keyboard.js:1648 (7fd1da288040 @ 46)
#1   7ffd86d1de70 b   /media/M2/GNOME/gnome-shell/js/ui/keyboard.js:1596 (7fd1da287ca0 @ 39)
#2   7ffd86d1ea80 b   /media/M2/GNOME/gnome-shell/js/ui/keyboard.js:1225 (7fd1da2858b0 @ 167)
#3   7ffd86d1f480 I   self-hosted:979 (7fd1da2515e0 @ 440)
#4   55f801681958 i   /media/M2/GNOME/gnome-shell/js/ui/main.js:493 (7fd1da9d88b0 @ 341)
#5   55f8016818a8 i   /media/M2/GNOME/gnome-shell/js/ui/grabHelper.js:215 (7fd1da225280 @ 100)
#6   55f8016817e0 i   /media/M2/GNOME/gnome-shell/js/ui/grabHelper.js:262 (7fd1da2253a0 @ 321)
#7   55f801681758 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:1259 (7fd1da2224c0 @ 179)
#8   7ffd86d222f0 I   self-hosted:979 (7fd1da2515e0 @ 440)
#9   7ffd86d223e0 b   resource:///org/gnome/gjs/modules/signals.js:142 (7fd1da9cb8b0 @ 386)
#10   55f8016816d0 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:866 (7fd1da21fd30 @ 148)
#11   55f801681648 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:509 (7fd1da21ea60 @ 65)
#12   55f8016815c0 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:545 (7fd1da21ed30 @ 51)
#13   7ffd86d24110 b   resource:///org/gnome/gjs/modules/signals.js:142 (7fd1da9cb8b0 @ 386)
#14   55f801681530 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:167 (7fd1da21c5e0 @ 25)
#15   55f8016814a8 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:343 (7fd1da21cee0 @ 132)
#16   55f801681420 i   /media/M2/GNOME/gnome-shell/js/ui/popupMenu.js:118 (7fd1da21c280 @ 45)
#17   7ffd86d25da0 I   self-hosted:979 (7fd1da2515e0 @ 440)