Making sure parent exists before providing its rendergroup.

If no parent is defined, calling the renderGroup will throw an
exception. This simply make sure that the variable exists. If it
doesn't, undefined will be sent, which is a value the function called
can handle.

src/Canvas2d/babylon.group2d.ts

@@ -803,7 +803,7 @@
 
             if (!rd._cacheNode) {
                 // Check if we have to allocate a rendering zone in the global cache texture
-                var res = this.owner._allocateGroupCache(this, this.parent.renderGroup, curWidth ? new Size(curWidth, curHeight) : null, rd._useMipMap, rd._anisotropicLevel);
+                var res = this.owner._allocateGroupCache(this, this.parent && this.parent.renderGroup, curWidth ? new Size(curWidth, curHeight) : null, rd._useMipMap, rd._anisotropicLevel);
                 rd._cacheNode = res.node;
                 rd._cacheTexture = res.texture;
                 rd._cacheRenderSprite = res.sprite;