fix the crash lead by the NUIBarButtonItemRenderer bug

Rendering the UIBarButton twice would lead to a crash " Exception: replaced layer <CAGradientLayer: 0x17ece3f0> is not a sublayer of <CALayer: 0x19068570>".

After investigation, I found that the issue is in the line 35:
`[layer insertSublayer:gradientLayer atIndex:backgroundLayerIndex];` 
It's trying to replace a sublayer of the `layer`, which is newly generated at line 23: `CALayer *layer = [CALayer layer];`, which doesn't have any sublayer. That leads to the crash.