UPDATE:
I decided to create a more modern version of the IFrame widget. This one is called IFrame Component:
Problem is that it used a deprecated function mendix.lang.nullExec, deprecated in MX9: https://docs.mendix.com/releasenotes/studio-pro/9.0#breaking-changes.
I created a new version 3.1.2 which can be downloaded in the Market Place
Since you can reproduce it, it worked in 9.0.2, you have a solid testcase, I think you have run into a bug that can get fixed best by Mendix. Create a support ticket for it via support.mendix.com