fix: Dodge SSR issue (#119)

Author: oliviertassinari

src/Popper.js

@@ -21,7 +21,8 @@ class Popper extends Component {
     modifiers: PropTypes.object,
     children: PropTypes.oneOfType([PropTypes.node, PropTypes.func]),
     target: PropTypes.oneOfType([
-      PropTypes.instanceOf(Element),
+      // the following check is needed for SSR
+      PropTypes.instanceOf(typeof Element !== 'undefined' ? Element : Object),
       PropTypes.shape({
         getBoundingClientRect: PropTypes.func.isRequired,
         clientWidth: PropTypes.number.isRequired,