I dug into the code and found that WP is looking for a field which matches this jQuery selector input[id*="-title"] when appending the widget title, so you have two choices:
1) Change the key of the custom field so that it ends in the string “-title”. This only seems possible if you’re registering your custom fields with code, not through the admin interface.
2) Add the title field manually in your widget’s form() method (and widget() method if you’re displaying it on the frontend). I reused some code I found in default-widgets with good results.
Good luck,
Marc
