I made some minor improvements to your form.php to see the choosen Posttitle within the widget-dashboard, also with closed widget forms.
Have a look at this gist.
Have a look at the lines 9 & 10
We don’t want an empty dropdown here with files attachements, so I suggest to remove it by default.
Now, the line 23
I added a hidden input-form-field to keep the title of the choosen post or page. For the WordPress Widget Class it’s important to have a field named explicity title to generate the in-widget-title from.
Last, but not least – lines 49 to 57
This small piece of jQuery grabs the Posttitle from the selected option and returns it to our introduced hidden input.