code: def test_get_variable_nodes_raises_error(self):
      self.graph.add_edges_from(
          [("a", "phi1"), ("b", "phi1"), ("b", "phi2"), ("c", "phi2")]
      )
      self.assertRaises(ValueError, self.graph.get_variable_nodes)
  
imports: import numpy
  import unittest
  from pgmpy.factors.discrete import DiscreteFactor
  from pgmpy.models import FactorGraph
  from pgmpy.models import MarkovNetwork
  from pgmpy.models import JunctionTree
  from pgmpy.tests import help_functions
label_desc: Use image node_id 6 for calling the TestFactorGraphMethods obj's underlying member method code with example usage: obj.test_get_variable_nodes_raises_error() without return types