If you try to remove a class with a nonexistent datatype, eZ will crash with a fatal error.

This problably happens because "* ezpublish_legacy/kernel/classes/ezcontentclass.php#815*", removableInformation function

// Check class attributes
foreach ( $this->fetchAttributes() as $attribute )
{
	$dataType = $attribute->dataType();
	if ( !$dataType->isClassAttributeRemovable( $attribute ) )
	{
		$info = $dataType->classAttributeRemovableInformation( $attribute, $includeAll );
		$result['list'][] = $info;
		if ( !$includeAll )
			return $result;
	}
}

It shouldn't be an impediment to remove a class with a nonexistent datatype.

Steps to Reproduce

1. Create a custom datatype. A simple way to do this is to install eZComments package. eZComments will change the article class and add its own custom datatype.
2. Duplicate article class and name it "article_1".
3. Remove the "article_1" class and confirm there will be no problem.
4. Delete the ezcomments extension
5. Try to remove article class and you will see a fatal error since the the comments datatype does not exist.