Error when unmounting before loading is done

[Robinfr]

If you unmount the Spritesheet before the loading is done, an error is thrown.

You probably have to check whether the component is still mounted if the onload callback.

EDIT: this line is probably returning null since it has already been unmounted.

[danilosetra]

@Robinfr checkout this new version 👍 Also fix #19 . Thanks!

[Robinfr]

Will check it out, thanks!

[Robinfr]

Works great! 😄