From the documentation:

'Since the default groups (the "NULL group") name is NULL, it will not be returned with this functions'

This '[NULL]' group is returned by this function as the first group when reading the following configuration file:

==============================

viewer_size=60

[1]
type=network_load
cpu_no=-1
viewer_size=96
next_color=3
color=-16777040
use_background_color=false
interface=eth
interface_no=2
interface_direction=2
max=696772

[2]
type=network_load
interface=eth
interface_no=2
interface_direction=1
max=1
next_color=3
viewer_size=60
color=16711856

==============================