As we can read in this forum topic:
https://forum.qcad.org/t/tip-when-dealing-with-an-exception-occurred-entity-not-saved-xxxx-error-on-save/11970

The tool can report “Object selected: nnn” when selecting an offending entity while nothing seems to be selected.
The search for the offending entity can be simplified by also reporting its Block name.

    if (isEntity(obj)) {
        di.selectEntity(obj.getId());
        EAction.handleUserMessage(qsTr("Object selected:") + " " + obj.getId() + " (" + obj.getBlockName() + ")");
    }

The object ID nnn is of no use for a common user.

Automatically switching to the Block in question is perhaps even better.

In the referred topic the user explains how to find and delete the entity.
It won't be saved thus saving and reopening is then the more simplier solution.

Offending is for example an ellipse with a negative ratio.
Not automatically fixed by correctMajorMinor().
But easily fixed when selected (in the current Block).

Regards,
CVH