NPE when calling Local.stop() without providing BrowserStack key #35

When Local.stop() is called without having provided the BrowserStack key in the options, a NullPointerException results.

Expected Behavior

While I can see a case for requiring the BrowserStack key when stopping a BrowserStack instance (but only if it's actually used), if it is required but not provided, then a meaningful error message should be provided rather than a NPE with no message.

Actual Behavior

A NullPointerException is thrown, and there's no exception message.