Could not load any valid backend / no '_Rendered' object with forward operator

Describe the bug

I’m trying to simulate EEG data, for which I need to compute the forward operator with fsaverage. This however produced the following error:

RuntimeError: Could not load any valid 3D backend: ['mayavi', 'pyvista']

I then successfully installed pyvista manually, but installing mayavi generated a large error log. If I now try to compute the forward operator, I keep getting the following error: AttributeError: 'NoneType' object has no attribute '_Renderer'

Steps to reproduce

I copied the MNE tutorial, as such:

import os.path as op

import mne
from mne.datasets import eegbci
from mne.datasets import fetch_fsaverage

# Download fsaverage files
fs_dir = fetch_fsaverage(verbose=True)
subjects_dir = op.dirname(fs_dir)

# The files live in:
subject = 'fsaverage'
trans = 'fsaverage'  # MNE has a built-in fsaverage transformation
src = op.join(fs_dir, 'bem', 'fsaverage-ico-5-src.fif')
bem = op.join(fs_dir, 'bem', 'fsaverage-5120-5120-5120-bem-sol.fif')

---

raw_fname, = eegbci.load_data(subject=1, runs=[6])
raw = mne.io.read_raw_edf(raw_fname, preload=True)

# Clean channel names to be able to use a standard 1005 montage
new_names = dict(
    (ch_name,
     ch_name.rstrip('.').upper().replace('Z', 'z').replace('FP', 'Fp'))
    for ch_name in raw.ch_names)
raw.rename_channels(new_names)

# Read and set the EEG electrode locations
montage = mne.channels.make_standard_montage('standard_1005')

raw.set_montage(montage)
raw.set_eeg_reference(projection=True)  # needed for inverse modeling

# Check that the locations of EEG electrodes is correct with respect to MRI
mne.viz.plot_alignment(
    raw.info, src=src, eeg=['original', 'projected'], trans=trans,
    show_axes=True, mri_fiducials=True, dig='fiducials')

Expected results

I was expecting a plot and a base to compute the forward operator.

Actual results

EDF file detected
Setting channel info structure...
Creating raw.info structure...
Reading 0 ... 19999  =      0.000 ...   124.994 secs...
Adding average EEG reference projection.
1 projection items deactivated
Average reference projection was added, but has not been applied yet. Use the apply_proj method to apply it.
Reading /Users/[USER]/mne_data/MNE-fsaverage-data/fsaverage/bem/fsaverage-ico-5-src.fif...
Using outer_skin.surf for head surface.

---

<ipython-input-30-c1141462bfd6> in <module>
     18 mne.viz.plot_alignment(
     19     raw.info, src=src, eeg=['original', 'projected'], trans=trans,
---> 20     show_axes=True, mri_fiducials=True, dig='fiducials')

<decorator-gen-128> in plot_alignment(info, trans, subject, subjects_dir, surfaces, coord_frame, meg, eeg, fwd, dig, ecog, src, mri_fiducials, bem, seeg, fnirs, show_axes, fig, interaction, verbose)

~/anaconda3/envs/opencv/lib/python3.7/site-packages/mne/viz/_3d.py in plot_alignment(***failed resolving arguments***)
   1034 
   1035     # initialize figure
-> 1036     renderer = _get_renderer(fig, bgcolor=(0.5, 0.5, 0.5), size=(800, 800))
   1037     if interaction == 'terrain':
   1038         renderer.set_interactive()

~/anaconda3/envs/opencv/lib/python3.7/site-packages/mne/viz/backends/renderer.py in _get_renderer(*args, **kwargs)
     31 def _get_renderer(*args, **kwargs):
     32     set_3d_backend(get_3d_backend(), verbose=False)
---> 33     return backend._Renderer(*args, **kwargs)
     34 
     35 

AttributeError: 'NoneType' object has no attribute '_Renderer'

Additional information

Platform:      Darwin-19.4.0-x86_64-i386-64bit
Python:        3.7.7 (default, Mar 26 2020, 10:32:53)  [Clang 4.0.1 (tags/RELEASE_401/final)]
Executable:    /Users/[USER]/anaconda3/envs/opencv/bin/python
CPU:           i386: 4 cores
Memory:        Unavailable (requires "psutil" package)
mne:           0.20.0
numpy:         1.18.1 {blas=mkl_rt, lapack=mkl_rt}
scipy:         1.4.1
matplotlib:    3.2.1 {backend=module://ipykernel.pylab.backend_inline}

sklearn:       0.22.1
numba:         Not found
nibabel:       Not found
cupy:          Not found
pandas:        1.0.3
dipy:          Not found
mayavi:        Not found
pyvista:       0.24.1
vtk:           8.1.2

[GuillaumeFavelier]

Thanks for the quick response.

Perhaps the backend had to be set just once.

I think so.

Feel free to close if everything works for you now 👍

[DanielvdC]

After restarting my session the code actually works without the snippet as well. Perhaps the backend had to be set just once.