Fix Aside doc for undefined record

Closes #2511

docs/CreateEdit.md

@@ -164,14 +164,18 @@ The `aside` component receives the same props as the `Edit` or `Create` child co
 const Aside = ({ record }) => (
     <div style={{ width: 200, margin: '1em' }}>
         <Typography variant="title">Post details</Typography>
-        <Typography variant="body1">
-            Creation date: {record.createdAt}
-        </Typography>
+        {record && (
+            <Typography variant="body1">
+                Creation date: {record.createdAt}
+            </Typography>
+        )}
     </div>
 );
 ```
 {% endraw %}
 
+**Tip**: Always test that the `record` is defined before using it, as react-admin starts rendering the UI before the API call is over.
+
 ## Prefilling a `<Create>` Record
 
 You may need to prepopulate a record based on another one. For that use case, use the `<CloneButton>` component. It expects a `record` and a `basePath` (usually injected to children of `<Datagrid>`, `<SimpleForm>`, `<SimpleShowLayout>`, etc.), so it's as simple to use as a regular field or input.

docs/Show.md

@@ -145,14 +145,18 @@ The `aside` component receives the same props as the `Show` child component: `ba
 const Aside = ({ record }) => (
     <div style={{ width: 200, margin: '1em' }}>
         <Typography variant="title">Post details</Typography>
-        <Typography variant="body1">
-            Creation date: {record.createdAt}
-        </Typography>
+        {record && (
+            <Typography variant="body1">
+                Creation date: {record.createdAt}
+            </Typography>
+        )}
     </div>
 );
 ```
 {% endraw %}
 
+**Tip**: Always test that the `record` is defined before using it, as react-admin starts rendering the UI before the API call is over.
+
 ## The `<ShowGuesser>` component
 
 Instead of a custom `Show`, you can use the `ShowGuesser` to determine which fields to use based on the data returned by the API.