==================================================================
The problem:
OriginGroupExtension::OriginGroupExtension () {
initExtensionType(OriginGroupExtension::getExtensionClassTypeId());
EXTENSION_ADD_PROPERTY_TYPE ( Origin, (0), 0, App::Prop_Hidden, "Origin linked to the group" );
}
initializes the Origin property to null.
Then Document::Restore => readObjects => addObject => slotNewObject => ViewProviderDocumentObject::updateView => ViewProviderBody::updateData => ViewProviderGeometryObject::updateData =>
ViewProviderDocumentObject::updateData => ViewProvider::updateData => ViewProviderOriginGroupExtension::extensionUpdateData => updateOriginSize() => OriginGroupExtension::getOrigin,
the latter throws an exception because the origin property is null.
Afterwards, the origin property is initialized during Document::Restore => readObjects => ExtensionContainer::Restore() => PropertyContainer::Restore() => App::PropertyLink::Restore()
=> App::PropertyLink::setValue() => Property::hasSetValue, which triggers an onChange: Body::onChanged => BodyBase::onChanged => Feature::onChanged => DocumentObject::onChanged =>
Document::onChangedProperty => Document::slotChangedObject => ViewProviderOriginGroupExtension::slotChangedObjectGui => ViewProviderOriginGroupExtension::updateOriginSize =>
OriginGroupExtension::getOrigin,
now, the latter that is the same that was throwing the exception and generating the error message above, does not throw anymore because Origin has been initialized.
When creating a new object, isNew==true; whereas when loading a file, isNew==false. Therefore, when loading a file setupObject is not executed. SetupObject, effectively initializes
the extension via: Body::setupObject => DocumentObject::setupObject => OriginGroupExtension::onExtendedSetupObject.
DocumentObject * Document::addObject(const char* sType, const char* pObjectName, bool isNew)
{
[more code here]
// Call the object-specific initialization
if (!d->undoing && !d->rollback && isNew) {
pcObject->setupObject ();
}
[more code here]
As DocumentObject code is generic for all objects (workbenches), by design it was chosen to initialize the object only if new. Therefore a object or an extension of a object being restored,
between the addition (addObject) and the restoring of the properties, must by design expect it not to be initialized.
The solution:
Making use of the Restore flag at document level (the one at object level is not sufficient), no exception regarding the null property is reported during document restoring.
Ticket:
https://freecadweb.org/tracker/view.php?id=2530fixes#2530
It seems on some system calling QTreeWidgetItem::takeChildren and then
addChild back is expensive. This fix avoids that but still keeps track
of item order in claimed children
The problem occurs when a child object is no longer claimed by its
former parent. The child tree item is not added back to Document root
even if it is the only instance left, which resulting the child object
disappearing entirely from the tree view.
