hide() throw exception when strip is closed already

[baoshan]

Unless there's a method to test if Strip is closed (or shown), hide() should not throw exception when closed already.

null is not an object (evaluating 'this.view.options')

From https://github.com/staaky/strip/blob/master/src/js/window.js#L430

[staaky]

Thanks, fixed this one and the other bug you found in 1.6.2