fix: check for el before resetSourceSet

[brandonocasey]

Fixes #5175

[thijstriemstra]

Can this be reproduced in a test (that can verify dispose better)? Because I've seen other issues with dispose, while writing unit tests for a plugin, and having a test that catches them all would be nice. Basically a test that does not wait for the player html elements to tear down; so once created, dispose and go to next test (triggering these cleanup issues).

A call to dispose should basically never trigger an error IMO, and this fix does not guarantee prevention of introducing similar bugs because there's no test reproducing this.

[brandonocasey]

Going to make a new pr with the suggested unit test @thijstriemstra, as we do want to prevent these issues in the future 👍

[brandonocasey]

PR for reference #5179

[thijstriemstra]

awesome @brandonocasey