fix: an issue that crashes when cancel is not present

[ajakka]

I have a use case where the user swipes multiple pdf files, and when the pdf is still loading and user swipes it, the cancel function get fired before its initialized. I fixed it by checking if the cancel function exists before calling it.

[ajakka]

@wonday could you please merge this asap, it's a simple change yet it crashes apps in production, I have to manually change it in node_modules every time a yarn install is invoked.