Delay focus on draftail widget for 50ms

This avoids an IndexSizeError on IE11 https://github.com/wagtail/wagtail/pull/6968#issuecomment-822591161 , apparently due to the focus triggering a field update before it has been fully initialised.

client/src/entrypoints/admin/telepath/widgets.js

@@ -155,7 +155,9 @@ class DraftailRichTextArea {
                 return result;
             },
             focus: () => {
-                input.draftailEditor.focus();
+                setTimeout(() => {
+                    input.draftailEditor.focus();
+                }, 50);
             },
         };
     }