fix(DatePicker): prevent TypeError on mobile (#1321)
fix(DatePicker): prevent TypeError on mobile (#1321)

When flatpicker detects a mobile device, it doesn't render its own
elements.
When Carbon tries to add its custom classes to those non-existing
elements, an TypeError (`calendarContainer is undefined`) occurred.
Now we check if the elements exist first, before adding classes.

Fixes #1305.
carbon-design-system/carbon-components-react,carbon-design-system/carbon-components,carbon-design-system/carbon-components,carbon-design-system/carbon-components,joshblack/carbon-components-react,carbon-design-system/carbon-components-react,joshblack/carbon-components-react