fix(StatusAlert): Fix focus error for dismissible alert. (#89)

StatusAlert requires a check for existence of xButton in componentDidUpdate to prevent errors when
displaying dismissible alerts.

src/StatusAlert/index.jsx

@@ -32,7 +32,7 @@ class StatusAlert extends React.Component {
   }
 
   componentDidUpdate(prevState) {
-    if (this.state.open && !prevState.open) {
+    if (this.state.open && !prevState.open && this.xButton) {
       this.xButton.focus();
     }
   }