code: def test_assert_univariate(self):
      with pytest.raises(AssertionError):
          self.series1._assert_univariate()
      self.series1.univariate_component(0)._assert_univariate()
  
imports: import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.tests.test_timeseries import TestTimeSeries
label_desc: Use image node_id 18 for calling the TestTimeSeriesMultivariate obj's underlying member method code with example usage: obj.test_assert_univariate() without return types