I'm assuming the pop up modal in the widget is being implemented as an executed callback using $core_v2_widget.GetExecutedFileUrl().
I can recreate an error with usage of $core_v2_editor within an executed callbacks, and I am logging a bug internally to address this. The cause seems to be related to the fact that it currently relies on the HttpContext's response, which no longer exists at the time of the request against the callback.
Unfortunately, there is not a an easy workaround at this time other than potentially implementing your own modal solution instead that doesn't rely on an iframe pointing at an executed velocity callback. It could be rendered inline in the widget, hidden by default, and displayed via CSS/JS instead.