Null check node before we pass it to velocity

As presumably sometimes the node wasn't actually in the DOM for
whatever reason, so don't propagate exceptions out into the app.

Should fix https://github.com/vector-im/riot-web/issues/6515

src/Velociraptor.js

@@ -147,7 +147,7 @@ module.exports = React.createClass({
             // creating/destroying large numbers of elements"
             // (https://github.com/julianshapiro/velocity/issues/47)
             const domNode = ReactDom.findDOMNode(this.nodes[k]);
-            Velocity.Utilities.removeData(domNode);
+            if (domNode) Velocity.Utilities.removeData(domNode);
         }
         this.nodes[k] = node;
     },