When an entity has an optional association on a given field and also a column definition on that field, loading that entity generates a notice in UnitOfWork.php on line 1884. Other than the notice, the code works fine. This is the line:
$joinColumnValue = $data[$srcColumn];
$joinColumnValue = isset($data[$srcColumn]) ? $data[$srcColumn] : null;
I have attached a test case which reproduces the issue.