Fix the swift runtime failure on getImagePathFromUIImage

[denisbevilacqua]

Motivation:

When thumbnail is null (It happens in some videos), the following error message is displayed in the Xcode console:

Swift runtime failure: Unexpectedly found nil while unwrapping an Optional value'.

Solution:

This simple approach only checks that the thumbnail is not nil before calling the getImagePathFromUIImage method. There might be a better way to solve it, but this way fixes the issue.