In ObjectHydrator->_getEntity(...) looking for the discriminator should be in uppercase

On line 204:
$discrColumn = $this->_rsm->metaMappings[$this->_rsm->discriminatorColumns[$dqlAlias]];

$this->_rsm->discriminatorColumns[$dqlAlias] is in lowercase, whereas $this->_rsm->metaMappings[$this->_rsm->discriminatorColumns[$dqlAlias]] is in uppercase.

$discrColumn cannot be assigned.

A simple fix could be:
$discrColumn = $this->_rsm->metaMappings[strtoupper($this->_rsm->discriminatorColumns[$dqlAlias])];