Fix bug in react-tree-walker call

By default react-tree-walker passes in an undefined context, which
breaks packages like @material-ui and JSSProvider.  This fix passes
in an empty object for context, which fixes components that use the
legacy context API.

src/hocs/app-with-translation.js

@@ -82,7 +82,7 @@ export default function (WrappedComponent) {
           if (instance && instance.props && instance.props.ns) {
             nsFromTree = [...new Set(nsFromTree.concat(instance.props.ns))]
           }
-        })
+        }, {})
       }
 
       // Step 3: Perform data fetching, depending on environment