changing mode to Static or Kinematic of RigidBody causes runtime error

[volzhs]

Godot version:

Godot 3.0 RC1 05b1843

OS/device including version:

Ubuntu 17.10

Issue description:

changing mode to Static or Kinematic of RigidBody causes runtime error with bullet physics

0:00:00:0183 - Condition ' PhysicsServer::BODY_MODE_RIGID != mode && PhysicsServer::BODY_MODE_CHARACTER != mode ' is true.
----------
Type:Error
Description: 
Time: 0:00:00:0183
C Error: Condition ' PhysicsServer::BODY_MODE_RIGID != mode && PhysicsServer::BODY_MODE_CHARACTER != mode ' is true.
C Source: modules/bullet/rigid_body_bullet.cpp:958
C Function: _internal_set_mass

Steps to reproduce:

1. Ensure current physics 3d engine is Default or Bullet
2. Add RigidBody to scene
3. Change 'Mode' to Static or Kinematic
4. Run scene

Minimal reproduction project:

Test Physics 1.zip

[akien-mga]

CC @AndreaCatania

[AndreaCatania]

This is a fix for this Issue #15743