Editor crashes on shutdown when fetching a singleton via Engine

### Godot version

4.6.beta2

### godot-cpp version

4.5.0.stable sync and b724219737883e311f4cc4da1b49a0960dd00d38

### System information

Windows 11. I should note that on Linux, I do not observe this behavior, and unfortunately, I do not have a MacOS system to verify whether this problem exists there or not.

### Issue description

When using the following logic in an empty GDExtension:
```
void initialize_extension_module(ModuleInitializationLevel p_level) {
if (p_level == MODULE_INITIALIZATION_LEVEL_EDITOR) {
for (const String& singleton_name : Engine::get_singleton()->get_singleton_list()) {
Object* o = Engine::get_singleton()->get_singleton(singleton_name);
}
}
}
```
On Windows, when the editor closes, this leads to
```
[0]
[1] Object::~Object (core\object\object.cpp:2436)
[2] GDExtensionManager::`scalar deleting destructor`
[3] unregister_core_types (core\register_core_types.cpp:427)
[4] Main::cleanup (main\main.cpp:5229)
[5] widechar_main (platform\windows\godot_windows.cpp:105)
[6] _main (platform\windows\godot_windows.cpp:128)
[7] main (platform\windows\godot_windows.cpp:140)
```

### Steps to reproduce

See code snippet above.

### Minimal reproduction project

N/A