[3.x] Task.Delay throws NullReferenceException on HTML5 exports

[Bip901]

Tested versions

Reproducible in: v3.6.stable.mono.official [de2f0f1]

• Occurs both when "Export as Debug" is set and when it isn't.
• Does not occur on Windows exports

System information

Windows 11 64-bit, Chrome 131.0.6778.205

Issue description

Awaiting Task.Delay works fine in editor builds, but crashes on HTML5 exports, showing the following message on the console (F12):

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at System.Threading.WasmRuntime.TimeoutCallback (System.Int32 id) <0x2f69080 + 0x0000a> in :0
timeout callback threw a System.NullReferenceException

Steps to reproduce

• Open the attached minimal reproduction project in Godot
• It contains a single script:

public override void _Ready() {
    Task.Run(async () => {
        GD.Print("Waiting...");
        await Task.Delay(1000);
        GD.Print("Done");
    });
}

• Observe how it runs as expected in the editor
• Export to HTML5, then run python -m http.server when cd-d into the exported directory
• Browse http://localhost:8000, press F12 to keep the console window open
• Click TaskDelayDemo.html and observe the errors in the console, and that "Done" is never printed

Minimal reproduction project (MRP)

TaskDelayDemo.zip