Fix issue #523 - "Slider cannot be initialized in iframe"

[jayelkaake]

How to recreate the issue

Load glide in an iframe

What I Did

I changed the exist() method to use duck-typing instead of a null check.

The work that @ericmorand did in pull request #669 did not fully solve the problem, and also sort of makes it possible to pass invalid objects, so was not a preferred solution. Thanks for leading me in the right direction though @ericmorand!

How was it tested

Loaded in iframe and out of iframe.

[jedrzejchalubek]

Should be fixed with v3.6.2