Unable to inherit from ProjectSettings

[Shatur]

Godot version:
3.2.1

OS/device including version:
Arch Linux

Issue description:
If I trying to extend ProjectSettings I have parsing error:

modules/gdscript/gdscript_compiler.cpp:1864 - Condition "native.is_null()" is true. Returned: ERR_BUG

Extending ProjectSettings can be very useful to add helper methods such as add_gravity() instead of using get_setting("physics/3d/default_gravity") every time.

Steps to reproduce:

1. Create a script.
2. Extend it from ProjectSettings.
3. Add any method or variable.
4. Try to use it in code.

Minimal reproduction project:
PrjectSettings.zip
In this project I created MySettingsNode class that inherits from Node and works as expected. Also I created MySettingsProjectSettings class that have the same code, but will not work (just try to instantiate it from code to see the error.

Temporary workaround:
For now it is possible to create a class and with static functions and just access to all settings via ProjectsSettings because this is a singleton. This is useful to the error looks like a bug.