Only hide sensitive image if we can access it

To trigger the bug:

1. Sensitive image in album with four images.
2. Show sensitive image.
3. Swipe right to fourth image.
4. Wait the five seconds.

By swiping that far right, the ViewPager seems to have claimed the_image. If, after five seconds, we try to modify the_image, it is null.

Let's do two things:

• Replace the Kotlin coroutine with a Handler. That way we don't have to explicitly sleep.
• Check for the_image before using it.

Closes #53 (closed).