Calloutwidget - delete does not work after callout has been used

[zmetcalf]

Create and save a callout
Add the callout to page.
Commit page
Delete callout from page
Delete callout
Error comes up:

Cannot delete callout

Deleting the callout 'Test Callout' would require deleting the following protected related objects:

    Callout widget: Test Callout

What would the ramifications be if we changed this line: https://github.com/fusionbox/django-widgy/blob/master/widgy/contrib/page_builder/models.py#L176
to:

on_delete=models.SET_NULL)

Or is there a better fix?