Fatal error when using top / bottom widgets with PHP 7.1

The error in the log is this:

Error: [] operator not supported for strings in _views_slideshow_preprocess_views_slideshow() (line 143 of /{path_to_site}/sites/all/modules/contrib/views_slideshow/theme/views_slideshow.theme.inc).

If you look at the PHP documentation on array assignments, it says this:

Note: As of PHP 7.1.0, applying the empty index operator on a string throws a fatal error. Formerly, the string was silently converted to an array.

Put another way, you can look at the "backwards incompatibility" page of the release notes.

The empty index operator is not supported for strings anymore
Applying the empty index operator to a string (e.g. $str[] = $x) throws a fatal error instead of converting silently to array.

So you have to set arrays to be arrays, not assume they'll be converted silently into arrays when they're needed for that.

I'll upload a patch shortly that fixes this.