`ScriptExtension` produces missing virtual error on editor run

[voidedWarranties]

Godot version

v4.1.1.stable.official [bd6af8e]

System information

Godot v4.1.1.stable - Fedora Linux 38 (Workstation Edition) - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2060 (nvidia; 535.54.03) - AMD Ryzen 7 3700X 8-Core Processor (16 Threads)

Issue description

When starting a project that utilizes ScriptExtension with the editor, the following error occurs:

E 0:00:00:0703   _gdvirtual__get_language_call: Required virtual method ScriptExtension::_get_language must be overridden before calling.
  <C++ Source>   ./core/object/script_language_extension.h:106 @ _gdvirtual__get_language_call()

even though the method is implemented and registered.

From my debugging, I believe this is the condition under which the error happens:

1. Project starts from editor.
2. Godot attempts to load a script.
3. When the Script is constructed, Wrapped's constructor creates the Godot-side object, postinitialize_handler is called from the Godot side, and NOTIFICATION_POSTINITIALIZE is fired.
4. Script's notification handler fires, which attempts to call get_language.
5. The virtual call attempts to find the Script's corresponding extension. It cannot find it, so the call fails and the error is printed.
6. postinitialize_handler is called on the GDExtension side and the Script's extension is finally set.
7. Virtual calls outside of postinitialize_handler are successful.

I'm not 100% sure this is correct, but it seems fairly likely.

Steps to reproduce

1. Install any extension that utilizes ScriptExtension.
2. Create a script.
3. Run the project from the editor.

Minimal reproduction project

MRP_ScriptExtension_virtual.zip

I've attached binaries from my implementation as I don't think a different extension will produce a different result. You can verify that get_language is implemented here.

Usage:

1. Unzip the archive and start the editor.
2. The editor will crash on import due to an unrelated issue.
3. Open the editor again.
4. Start the main scene.

[Naros]

I can also reproduce this on the latest 4.2 dev builds.

[tarragonfly]

Reproducable in Godot 4.2.1 stable.

[Naros]

Fixed in #90776.