altTab: fix destroy-without-showing case

If the switcher is destroyed without ever being fully shown (either
because it couldn't get a keyboard grab, or just because there are no
apps to display), destroy it immediately rather than tweening it
towards destruction, since its contents haven't been built yet and
_allocate() will throw errors if it runs.

js/ui/altTab.js

@@ -34,7 +34,8 @@ function AltTabPopup() {
 AltTabPopup.prototype = {
     _init : function() {
         this.actor = new Shell.GenericContainer({ name: 'altTabPopup',
-                                                    reactive: true });
+                                                  reactive: true,
+                                                  visible: false });
 
         this.actor.connect('get-preferred-width', Lang.bind(this, this._getPreferredWidth));
         this.actor.connect('get-preferred-height', Lang.bind(this, this._getPreferredHeight));
@@ -365,6 +366,7 @@ AltTabPopup.prototype = {
     },
 
     destroy : function() {
+        if (this.actor.visible) {
             Tweener.addTween(this.actor,
                              { opacity: 0,
                                time: POPUP_FADE_TIME,
@@ -374,6 +376,8 @@ AltTabPopup.prototype = {
                                        this.actor.destroy();
                                    })
                              });
+        } else
+            this.actor.destroy();
     },
 
     _onDestroy : function() {