When changing from one modal to another, remove and add is-active-class ...

...properly.

If you change directly from one modal, say, via a link within it, to another modal, the class "is-active" stays on the old modalElement and isn't added to the new one. This is partly due to the if-condition regarding the "has-overlay" class of the html element. I put this into a nested if-statement. 
Additionally I wrote a few lines to remove "is-active" from the old modalElement (if it exists).