Remove instance check

[Esildor]

Once this PR is merged, this check will no longer be required, since a new instance will always be created.

[Guillemsc]

Hey again @Esildor.

What this check is preventing is calls to Add when the engine is shutting down (basically calls on OnDestroy). Without this check, if you play a tween at OnDestroy when the engine/game is being closed, it will throw an error telling you that you can't create GameObjects when the engine is shutting down.

Thus, I whink this check is still necessary and I would like to keep it, since as far as I know it does not interfeer with the issue you raisied with domain reload.