All Entity.parent to be set to undefined.

This should have always worked, but we were missing a `defined` check.

Reported via the [forum](https://groups.google.com/d/msg/cesium-dev/danz-dtw8Jg/Id6DBZwaEQAJ)

CHANGES.md

@@ -6,6 +6,7 @@ Change Log
 * Fixed an issue where the sun texture is not generated correctly on some mobile devices. [#3141](https://github.com/AnalyticalGraphicsInc/cesium/issues/3141)
 * Fixed a bug in the deprecated `jsonp` that prevented it from returning a promise.  Its replacement, `loadJsonp`, was unaffected.
 * Fixed glTF implementation to read the version as a string as per the specification and to correctly handle backwards compatibility for axis-angle rotations in glTF 0.8 models.
+* Fixed a bug that caused setting `Entity.parent` to `undefined` to throw an exception. [#3169](https://github.com/AnalyticalGraphicsInc/cesium/issues/3169)
 * Added `Model.maximumScale` and `ModelGraphics.maximumScale` properties, giving an upper limit for minimumPixelSize.
 
 ### 1.15 - 2015-11-02

Source/DataSources/Entity.js

@@ -297,7 +297,9 @@ define([
                 }
 
                 this._parent = value;
-                value._children.push(this);
+                if (defined(value)) {
+                    value._children.push(this);
+                }
 
                 var isShowing = this.isShowing;
 

Specs/DataSources/EntitySpec.js

@@ -459,4 +459,21 @@ defineSuite([
         expect(entity.show).toBe(true);
         expect(entity.isShowing).toBe(false);
     });
+
+    it('isShowing works when removing parent.', function() {
+        var entity = new Entity();
+        entity.parent = new Entity({
+            show : false
+        });
+        expect(entity.isShowing).toBe(false);
+
+        var listener = jasmine.createSpy('listener');
+        entity.definitionChanged.addEventListener(listener);
+
+        entity.parent = undefined;
+
+        expect(listener.calls.count()).toBe(2);
+        expect(listener.calls.argsFor(0)).toEqual([entity, 'isShowing', true, false]);
+        expect(entity.isShowing).toBe(true);
+    });
 });