app-display: Only reset filters when they have been initialized

The view might get mapped before the filters have been added, so
trying to reset to the "All" filter will throw an exception. Fix
by only do the reset if the filters have been initialized.

js/ui/appDisplay.js

@@ -115,7 +115,7 @@ ViewByCategories.prototype = {
         // Always select the "All" filter when switching to the app view
         this.actor.connect('notify::mapped', Lang.bind(this,
             function() {
-                if (this.actor.mapped)
+                if (this.actor.mapped && this._allFilter)
                     this._selectCategory(-1);
             }));