Fixed a bug in the hide function.

(gnome-shell:1808): Gjs-WARNING **: JS ERROR: Exception in callback for signal: showing: TypeError: Main.overview._group is undefined
dashVisible<._hide@/home/axp/.local/share/gnome-shell/extensions/hide-dash@xenatt.github.com/extension.js:38
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
_emit@resource:///org/gnome/gjs/modules/signals.js:124
Overview<._animateVisible@resource:///org/gnome/shell/ui/overview.js:571
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
Overview<.show@resource:///org/gnome/shell/ui/overview.js:543
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
Overview<.toggle@resource:///org/gnome/shell/ui/overview.js:673
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
_initializeUI/<@resource:///org/gnome/shell/ui/main.js:166