Error when closing a project with a Joint3D with only a B node attached

[madmiraal]

Godot version:
master: 8d4698d
3.2: f93c2dd

OS/device including version:
Ubuntu 20.10

Issue description:
When closing a project with a Joint3D with only a B node attached, the following error is generated:

ERROR: A body connected to joints was removed.
   at: remove_rigid_body_constraints:  (modules/bullet/space_bullet.cpp:485)

and in 3.2:

ERROR: remove_rigid_body_constraints: A body connected to joints was removed.
   At: modules/bullet/space_bullet.cpp:505.

Steps to reproduce:
Create a Joint3D with only a B node. Run and close the project.

Minimal reproduction project:
4.0: Joint3DBug-4.0.zip
3.2: Joint3DBug-3.2.zip