docstr: Should return None if the structure model only contains isolates
docstr_tok: ["Should","return","None","if","the","structure","model","only","contains","isolates"]
code: def test_isolates(self):
      """Should return None if the structure model only contains isolates"""
  
      nodes = [1, 3, 5, 2, 7]
      sm = StructureModel()
      sm.add_nodes_from(nodes)
      assert sm.get_largest_subgraph() is None
  
imports: import pytest
  from networkx.exception import NodeNotFound
  from causalnex.structure import StructureModel
label_desc: Use image node_id 4 for calling the TestStructureModelGetLargestSubgraph obj's underlying member method code with example usage: obj.test_isolates() without return types