OnRectTransformDimensionsChange()->UpdateBounds(true)

[MonsterZhangChen]

在提交https://github.com/qiankanglai/LoopScrollRect/commit/629112ce5cb5afcc9214753e830597e961042013中，将UpdateBounds()放在OnRectTransformDimensionsChange()方法中调用，我将这次提交合并到我们的项目中，发生了严重bug。原因在于，UpdateBounds()最终会引起ClearTempPool()->prefabSource.ReturnObject()的调用，而prefabSource`在没有初始化时为空，具体错误日志如下：

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UI.LoopScrollRect.ClearTempPool () (at Assets/Plugins/CustomUGUI/LoopScrollRect/Runtime/LoopScrollRect.cs:61)
UnityEngine.UI.LoopVerticalScrollRect.UpdateItems (UnityEngine.Bounds& viewBounds, UnityEngine.Bounds& contentBounds) (at Assets/Plugins/CustomUGUI/LoopScrollRect/Runtime/LoopVerticalScrollRect.cs:178)
UnityEngine.UI.LoopScrollRectBase.UpdateBounds (System.Boolean updateItems) (at Assets/Plugins/CustomUGUI/LoopScrollRect/Runtime/LoopScrollRectBase.cs:2181)
UnityEngine.UI.LoopScrollRectBase.OnRectTransformDimensionsChange () (at Assets/Plugins/CustomUGUI/LoopScrollRect/Runtime/LoopScrollRectBase.cs:1988)

OnRectTransformDimensionsChange()的调用无法预测，它甚至能在修改一个非激活的游戏物体时触发，是否考虑加入保护机制，或者回滚这次提交。