Fix benign error in KB UI. (#329)
Fix benign error in KB UI. (#329)

When deleting characters from the search bar the update handler gets a
null item. Check it before dereferencing `.ref` on it.

For reference, the console error:

    TypeError: Cannot read property 'ref' of undefined
        at Object.self.selectedItemChange (kb.js:46)
        at fn (eval at compile (angular.js:14605), <anonymous>:4:318)
        at m.d.(:8080/kb/anonymous function) [as itemChange] (http://ajax.googleapis.com/ajax/libs/angularjs/1.5.7/angular.min.js:83:232)
        at D (angular-material.min.js:13)
        at N (angular-material.min.js:13)
        at m.$digest (angular.js:17286)
        at b.$apply (angular.js:17552)
        at Pg.$$debounceViewValueCommit (angular.js:27516)
        at Pg.$setViewValue (angular.js:27488)
        at HTMLInputElement.l (angular.js:23730)
google/sling,google/sling,google/sling,google/sling,google/sling,google/sling