def _is_expr(node):
    return len(_exprchars.intersection(node)) > 0
Gets the error message that is displayed in case the value is
dataset/ETHPy150Open OpenMDAO/OpenMDAO-Framework/openmdao.main/src/openmdao/main/depgraph.py/_is_expr--crossover with--dataset/ETHPy150Open rwl/muntjac/muntjac/data/validators/null_validator.py/NullValidator.getErrorMessage