Magento – Call to undefined method Magento AttributeMetadataConverter.php

The CustomerExtension is not a model and thus doesn't have any ability to save itself, it is basically just a data container for extension attributes on for the CustomerInterface. You would need to utilize the CustomerRepository in order to save the data.

In working with extension attributes you may have defined in a extension_attributes.xml more on DevDocs, here are some of the pieces involved:

// Get the entity Interface from the entity Repository
$entity = $this->entityRepositoryInterface->get($entityId);

// Get existing ExtensionAttributes or create one
$entityExtension = $entity->getExtensionAttributes();
if ($entityExtension == null) {
    $entityExtension = $this->entityExtensionFactory->create();
}

// Set a value on your attribute
$entityExtension->setMyAttribute($value);

// Set the ExtensionAttributes back to the entity
$entity->setExtensionAttributes($entityExtension);

// Save the entity using the repository
$this->entityRepository->save($entity);

go to Fatal error: Call to a member function getName() on a non-object in app/code/core/Mage/ImportExport/Model/Export/Entity/Product.php on line 173

replace :

$path[] = $collection->getItemById($structure[$i])->getName();

with:

if($collection->getItemById($structure[$i]))
$path[] = $collection->getItemById($structure[$i])->getName();

Hope it will help you.