Prevents accessing an empty optional

Fixes #2982

I've missed this scenario: 
- RichTextView gets instantiated from a storyboard. 
- The Nib-Loading-Mechanism will set the view's properties (backgroundColor)
- This happens before the `textView` gets alloc'ed
- As a result, an implicitly unwrapped optional is being accessed before it's ready.

Plus, the `backgroundColor` should not have custom getters and setters, since it's a UIView property.

/cc @aerych (Thanks!)