Finding a good default for node group previews in general turned out to be quite tricky (T92811, T93754). After talking in chat we found that it would be good to disable the default preview generation for now, because it is often more confusing than helpful.

Additionally, we should add an operator to create a preview by rendering the active object. This might require a small refactor, because then we don't have THE operator to generate preview, but potentially multiple operators in the future. This can be done in a separate patch.

The image shows the new (left) and old (right) automatic previews for node groups.