GDNative: exporting properties crashes Godot at exit.

[Bromeon]

See originating issue godot-rust/gdnative#706

Godot version:
3.2 branch, 311ca0c

OS/device including version:
Windows 10

Issue description:
I am using the Rust GDNative binding with a custom-built Godot version. Everything works fine, until I export a Rust field, to be accessible as a property from GDScript.

Then, the engine starts and appears to run normally, however it crashes on exit with the following log:

CrashHandlerException: Program crashed
Dumping the backtrace. Please include this when reporting the bug on https://github.com/godotengine/godot/issues
[0] <couldn't map PC to fn name>
[1] <couldn't map PC to fn name>
[2] NativeScriptLanguage::_unload_stuff (D:\godot\modules\gdnative\nativescript\nativescript.cpp:955)
[3] NativeScriptLanguage::finish (D:\godot\modules\gdnative\nativescript\nativescript.cpp:1077)
[4] ScriptServer::finish_languages (D:\godot\core\script_language.cpp:185)
[5] Main::cleanup (D:\godot\main\main.cpp:2247)
[6] widechar_main (D:\godot\platform\windows\godot_windows.cpp:164)
[7] _main (D:\godot\platform\windows\godot_windows.cpp:184)
[8] main (D:\godot\platform\windows\godot_windows.cpp:196)
[9] __scrt_common_main_seh (D:\agent\_work\9\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288)
[10] BaseThreadInitThunk
-- END OF BACKTRACE --

@toasteater had the suspicion it could be this commit (see link above):

Revision: 311ca0c6f23784dfa831d8f058a335f698dcc5ea
Author: geekrelief <[email protected]>
Date: 23.12.2020 15:23:41
Message:
Removes the gdnative library when no script (gdns) references it any longer. This enables hot reload for gdnative.

(cherry picked from commit cc5d8bb5adf46aef63828fb23095589e51cca783)
----
Modified: modules/gdnative/nativescript/nativescript.cpp

After building both this and the prior version, both toasteater and I can confirm that this is the first commit that leads to the crash.

Steps to reproduce:

Set up simplest possible GDNative Rust project, 1 single class (I used godot-rust's Hello World example).
Add an exported property to that class:

#[property]
my_property: u32

Launch Godot.
Exit... 💥

Minimal reproduction project:

My fork of godot-rust, crash branch

[akien-mga]

CC @geekrelief @godotengine/gdnative

[geekrelief]

Worked with @Bromeon and @toasteater on the fix.