viz.get_3d_backend() should return None if no 3d backend was found, instead of raising an exception

Describe the problem

Currently, mne.viz.get_3d_backend() will raise a RuntimeError if no usable backend has been found:

https://github.com/mne-tools/mne-python/blob/d0b8bcf5201d7b5c278e7410e4035dc5aead30b8/mne/viz/backends/renderer.py#L119-L147

Describe your solution

I believe it should return None instead: if there is currently no 3d backend, then, well, there is none 😉

Describe possible alternatives

I currently have to be ready to catch a RuntimeError when I simply want to query whether I can produce a 3d visualization or not. Background: I’m plotting STCs in environments that sometimes don’t have working pyvista or mayavi installations. I want to find out when that’s the case, and switch to using matplotlib instead.

Additional context

I checked how get_3d_backend() is currently used, and without exception, any use is a comparison likeif get_3d_backend() != 'pyvista':; therefore, returning None if no working backend was found should not break any existing code.

cc @GuillaumeFavelier

[GuillaumeFavelier]

I’ll open a PR to take care of it then 😃

[hoechenberger]

have_3d_backend() sounds good to me. So this function would return True is there is a valid 3d backend and False otherwise, right?

That would be the idea, yes 😃