Changes to JavaScript's Storage API cause exceptions to be thrown

[TheWitness]

Describe the bug

The most recent Storages API JavaScript library changes the way it handled unset storage location that instead of returning silently, it throws an error. Cacti does not check if a storage location is set, and this therefore generates errors.

To Reproduce

To reproduce, do the following

1. Clear all your local storage in Cacti

2. Turn on the Debug Console

3. Navigate to the index.php page

4. See error

Expected behavior

Cacti should not throw exceptions even when an API changes it behavior.