code: def test_gradient_set_none(self):
      layer = _HeatmapOptionsMixin(gradient=["blue", "red"])
      layer.gradient = None
      assert layer.gradient is None
  
imports: import unittest
  import pytest
  import traitlets
  from ..heatmap import _HeatmapOptionsMixin, heatmap_layer, Heatmap, WeightedHeatmap
  from ..geotraitlets import InvalidPointException, InvalidWeightException
label_desc: Use image node_id 3 for calling the TestHeatmapOptionsMixin obj's underlying member method code with example usage: obj.test_gradient_set_none() without return types