I have two calendar widgets that operate like this. I emailed the developer for one of them, and he said that it was not the widget causing this issue. I've cleared defaults for all my calendar apps and set them again, and I still get prompted. Tapping the plus button or the title bar of these widgets will open my default calendar app (Google Calendar), but Android does not seem to save my calendar choice when clicking the event. Any fix for this? I have an LG V30 on Oreo.