<｜begin▁of▁sentence｜>### Instruction:
Given an undirected graph with a set of edges, can you use NetworkX's dyad_census method to calculate the dyad census, and then access a particular dyad type ('null', 'mutual', or 'asymmetric') using the __getitem__ method?
### Response:
from networkx.algorithms import dyad_census

# Create a graph
g = nx.Graph()
g.add_edges_from([(1, 2), (2, 3), (1, 3), (3, 4)])

# Perform dyad census
result = dyad_census(g)

# Access the result for 'null' dyads
null_dyads = result['null']
print('Number of null dyads:', null_dyads)
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