This is indeed a bug which was introduced in version 5.17.0. The image widget did not pass its context when calling a microflow. The bug has been fixed in version 5.19.0, which is planned to be released on August 31st.
Ronald,
Is your entity perhaps Non-persistant? I got the same error, again, yesterday,and remembered from my last time it was both with non-persistant entities.
Making the entity persistable fixed it for me. Altough I doubt by the name of your entity that it would be non-persistant :-)