def get_root_node(ad_group_criterions):
    """
    Returns the root node of a tree. This operation assumes that a complete 
    product partition tree is provided for one ad group. The node that has
    null ParentCriterionId is the root node.

    :param ad_group_criterions: The ad group criterions that contain the product partition tree.
    :type ad_group_criterions: ArrayOfAdGroupCriterion
    :return: The ad group criterion that was added to the list of PartitionActions.
    :rtype: AdGroupCriterion

    """

    root_node=None
    for ad_group_criterion in ad_group_criterions['AdGroupCriterion']:
        if ad_group_criterion.Criterion.ParentCriterionId is None:
            root_node=ad_group_criterion
            break

    return root_node