Editor crashes on exit with extension loaded, after AHashMap sync (ce03221)

### Godot version

fb1821384

### godot-cpp version

ce03221

### System information

win11 9700x+5070ti

### Issue description

I ran into a crash after updating to ce03221.

Our extension registers a lot of classes, and now the editor consistently crashes at the very end of shutdown. Right before the crash I get: ERROR: Parameter "singleton" is null

followed by orphan StringName warnings (e.g. "Node"), then an access violation.

Going back to 193e164 fixes the crash.

### Steps to reproduce

godot --headless --editor --quit

### Minimal reproduction project

None

Start by running `godot --headless --editor --quit` with the extension loaded on godot-cpp ce03221, then compare the shutdown behavior with 193e164. Investigate the reported null singleton, orphan StringName warnings, and final access violation around editor exit. Done means the editor exits without the crash or shutdown errors.

Written by the indexing model from the issue text.