Gracefully handle undefined data in getTreeEntries()

app/assets/javascripts/diffs/workers/tree_worker.js

@@ -4,6 +4,11 @@ import { generateTreeList } from '../store/utils';
 // eslint-disable-next-line no-restricted-globals
 self.addEventListener('message', e => {
   const { data } = e;
+
+  if (data === undefined) {
+    return;
+  }
+
   const { treeEntries, tree } = generateTreeList(data);
 
   // eslint-disable-next-line no-restricted-globals