Add a null guard for serverConfig

This is often null while the component is on its first render, and is called during that render. It is eventually populated by React, and the function re-called - we just have to be patient.

src/components/structures/MatrixChat.js

@@ -204,7 +204,7 @@ export default React.createClass({
     },
 
     getFallbackHsUrl: function() {
-        if (this.props.serverConfig.isDefault) {
+        if (this.props.serverConfig && this.props.serverConfig.isDefault) {
             return this.props.config.fallback_hs_url;
         } else {
             return null;