get_nested() crashes on link kind = "erspan" (element of GRE)

[crosser]

$ sudo ip tunnel add greif mode gre remote 189.254.111.111 local 189.254.222.222 ttl 255

This operation creates interface kind="gre" plus an interface kind="erspan". When we get attributes for the latter, result contains IFLA_LINKINFO -> IFLA_INFO_DATA which is a string:

>>> [(el.get_attr("IFLA_IFNAME"), el.get_nested("IFLA_LINKINFO", "IFLA_INFO_KIND"), el.get_nested("IFLA_LINKINFO", "IFLA_INFO_DATA")) for el in IPRoute().get_links() if el.get_nested("IFLA_LINKINFO", "IFLA_INFO_KIND") == "erspan"]
[('erspan0', 'erspan', '05:00:16:00:01:00:00:00:08:00:15:00:00:00:00:00:08:00:01:00:00:00:00:00:06:00:02:00:00:00:00:00:06:00:03:00:20:00:00:00:08:00:04:00:00:00:00:00:08:00:05:00:00:00:00:00:08:00:06:00:00:00:00:00:08:00:07:00:00:00:00:00:05:00:08:00:00:00:00:00:05:00:09:00:00:00:00:00:05:00:0a:00:00:00:00:00:08:00:14:00:00:00:00:00:06:00:0e:00:00:00:00:00:06:00:10:00:00:00:00:00:06:00:11:00:00:00:00:00:06:00:0f:00:00:00:00:00:05:00:13:00:00:00:00:00')]
>>> 

For other types of interfaces, IFLA_INFO_DATA contains other useful nested attributes, so I want to be able to use, for instance,

get_nested("IFLA_LINKINFO", "IFLA_INFO_DATA", "IFLA_VRF_TABLE")

but when there is an erspan interface in the list, get_nested crashes:

>>> [(el.get_attr("IFLA_IFNAME"), el.get_nested("IFLA_LINKINFO", "IFLA_INFO_KIND"), el.get_nested("IFLA_LINKINFO", "IFLA_INFO_DATA", "IFLA_SOME_ATTRIBUTE")) for el in IPRoute().get_links() if el.get_nested("IFLA_LINKINFO", "IFLA_INFO_KIND") == "erspan"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
  File "/usr/lib/python3/dist-packages/pr2modules/netlink/__init__.py", line 1130, in get_nested
    pointer = pointer.get_attr(attr)
AttributeError: 'str' object has no attribute 'get_attr'
>>>
I understand that `get_nested()` is supposed to be "safe": never crash, and just return `None` if it cannot access an attribute.