guard against null keyEventTarget (#48)

* guard against null keyEventTarget

There are two calls to _listenKeyEventListeners() and one already guards against  null this.keyEventTarget. This change guards the call to _listenKeyEventListeners in attached from using a null keyEventTarget.

* Moving the keyEventTarget check

Moving the keyEventTarget check into _listenKeyEventListeners()

* test for fix to #49

* Removed blank line

* review nits

iron-a11y-keys-behavior.html

@@ -391,12 +391,15 @@
       _resetKeyEventListeners: function() {
         this._unlistenKeyEventListeners();
 
-        if (this.isAttached && this.keyEventTarget) {
+        if (this.isAttached) {
           this._listenKeyEventListeners();
         }
       },
 
       _listenKeyEventListeners: function() {
+        if (!this.keyEventTarget) {
+          return;
+        }
         Object.keys(this._keyBindings).forEach(function(eventName) {
           var keyBindings = this._keyBindings[eventName];
           var boundKeyHandler = this._onKeyBindingEvent.bind(this, keyBindings);

test/basic-test.html

@@ -431,6 +431,15 @@
     });
   });
 
+  suite('remove key behavior with null target', function () {
+    test('add and remove a iron-a11y-keys-behavior', function () {
+      var element = document.createElement('x-a11y-basic-keys');
+      element.keyEventTarget = null;
+      document.body.appendChild(element);
+      document.body.removeChild(element);
+    });
+  });
+
 });
   </script>
 </body>