fix: java.lang.NullPointerException

[nukc]

please see https://github.com/Piasy/BigImageViewer/issues/129

[Piasy]

I think FrescoImageViewFactory has the same problem, could you please fix it too?

[nukc]

I check it now

[nukc]

造成 null 的原因主要还是我这边是直接 new BigImageView(context); 也想过从构造方法入手，但想了下 mThumbnailView 并不是必须的也就不一定要初始化 mThumbnailScaleType, 但目前来说，直接判断 null 是最简单且有效的。

Google translate: The main reason for causing null is that I am use new BigImageView(context); I also thought about starting with the constructor, but I thought that mThumbnailView is not necessary and I don't have to initialize mThumbnailScaleType , but for now, it is the easiest and most effective to judge whether it is null or not.

[Piasy]

v1.5.6 is released, check it out!