Refactor `Modal.dispose` and add test

Created by: iamandrewluca

Having variables initialised from start _isTransitioning is better.
Would be better to add an no-undef eslint rule to check for undefined variables use.
Reordered variables by priority at show and hide function entrance.

• fix Modal.dispose method
• add test for Modal.dispose
• add _isTransitioning default value

no-undef - this rule is enabled by default, but does not what we want no-use-before-define - kind of does what we want, but does not work for class members

Resolves #27456 (closed)