fix broken GuideContent demos not rendering because of unexpected useEffect return value

demos/src/GuideContent/ExportHTML/React/index.jsx

@@ -16,7 +16,7 @@ export default () => {
 
   useEffect(() => {
     if (!editor) {
-      return null
+      return undefined
     }
 
     // Get the initial content …

demos/src/GuideContent/ExportJSON/React/index.jsx

@@ -16,7 +16,7 @@ export default () => {
 
   useEffect(() => {
     if (!editor) {
-      return null
+      return undefined
     }
 
     // Get the initial content …

demos/src/GuideContent/ReadOnly/React/index.jsx

@@ -20,7 +20,7 @@ export default () => {
 
   useEffect(() => {
     if (!editor) {
-      return null
+      return undefined
     }
 
     editor.setEditable(editable)