def get_parent_unit(xblock):
    """
    Finds xblock's parent unit if it exists.

    To find an xblock's parent unit, we traverse up the xblock's
    family tree until we find an xblock whose parent is a
    sequential xblock, which guarantees that the xblock is a unit.
    The `get_parent()` call on both the xblock and the parent block
    ensure that we don't accidentally return that a unit is its own
    parent unit.

    Returns:
        xblock: Returns the parent unit xblock if it exists.
        If no parent unit exists, returns None
    """
    while xblock:
        parent = xblock.get_parent()
        if parent is None:
            return None
        grandparent = parent.get_parent()
        if grandparent is None:
            return None
        if parent.category == "vertical" and grandparent.category == "sequential":
            return parent
        xblock = parent