AttributeError: 'open3d.open3d.geometry.VoxelGrid' object has no attribute 'ndim'

Hi, I am trying to convert mesh to voxel using VoxelGrid().
Code-

import open3d as o3d
import numpy as np
import matplotlib.pyplot as plt

input='obj_file_path'
mesh=o3d.io.read_triangle_mesh(input)
mesh.compute_vertex_normals()
print(mesh)
voxel=o3d.geometry.VoxelGrid.create_from_triangle_mesh(mesh,voxel_size=0.05)
print(voxel.has_voxels())
print(voxel)

fig = plt.figure()
ax = fig.gca(projection='3d')
ax.voxels(voxel)
ax.imshow(np.asarray(voxel),cmap='gray')
fig.savefig('save_voxel_to_visualize')

Output-
geometry::TriangleMesh with 49751 points and 100000 triangles.
True
geometry::VoxelGrid with 480 voxels.
AttributeError: 'open3d.open3d.geometry.VoxelGrid' object has no attribute 'ndim'

Is there any way to visualize and save the converted voxel in mat files??
Thanks!!