Panic when Artifact tool is not properly setup

Seems like this is something we could catch before reaching the terminal session, or at least provide an error instead of panicking

$ go run main.go                                                                                                                                                                                                                                                                      Agent!

User -> hello

Agent -> panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x18 pc=0x100a9eba0]

goroutine 1 [running]:
google.golang.org/adk/internal/toolinternal.(*internalArtifacts).List(0x10180c050?, {0x101c76638?, 0x140001f12c0?})
        <autogenerated>:1 +0x30
google.golang.org/adk/tool/loadartifactstool.(*artifactsTool).appendInitialInstructions(0x140001f1040?, {0x1012ad560, 0x140001f12c0}, 0x140001f1040)
        /Users/tony/go/pkg/mod/google.golang.org/adk@v0.1.0/tool/loadartifactstool/load_artifacts_tool.go:122 +0x68
google.golang.org/adk/tool/loadartifactstool.(*artifactsTool).ProcessRequest(0x140002a29a0, {0x1012ad560, 0x140001f12c0}, 0x140001f1040)
        /Users/tony/go/pkg/mod/google.golang.org/adk@v0.1.0/tool/loadartifactstool/load_artifacts_tool.go:115 +0x54
google.golang.org/adk/internal/llminternal.toolPreprocess({0x1012abba8, 0x14000103cb0}, 0x140001f1040, {0x140002a29c0?, 0x140001dd4d8?, 0x100502b08?})
        /Users/tony/go/pkg/mod/google.golang.org/adk@v0.1.0/internal/llminternal/base_flow.go:232 +0xdc
google.golang.org/adk/internal/llminternal.(*Flow).preprocess(0x140002872c0, {0x1012abba8, 0x14000103cb0}, 0x140001f1040)
        /Users/tony/go/pkg/mod/google.golang.org/adk@v0.1.0/internal/llminternal/base_flow.go:218 +0x300

...

package main

import (
	"context"
	"fmt"
	"log"
	"os"

	"google.golang.org/adk/agent/llmagent"
	"google.golang.org/adk/cmd/launcher/adk"
	"google.golang.org/adk/cmd/launcher/full"
	"google.golang.org/adk/model/gemini"
	"google.golang.org/adk/server/restapi/services"
	"google.golang.org/adk/tool"
	"google.golang.org/adk/tool/functiontool"
	"google.golang.org/adk/tool/loadartifactstool"
	"google.golang.org/genai"
)

func main() {
	fmt.Println("Agent!")

	ctx := context.Background()

	model, _ := gemini.NewModel(ctx, "gemini-2.5-flash", &genai.ClientConfig{
		APIKey: os.Getenv("GOOGLE_API_KEY"),
	})

	sumTool, err := makeSummer()
	if err != nil {
		log.Fatalf("Failed to create summer: %v", err)
	}

	agent, err := llmagent.New(llmagent.Config{
		Name:        "hello_time_agent",
		Model:       model,
		Description: "Tells the current time in a specified city.",
		Instruction: "You are a helpful assistant that tells the current time in a city.",
		Tools: []tool.Tool{
			loadartifactstool.New(),
			sumTool,
		},
	})
	if err != nil {
		log.Fatalf("Failed to create agent: %v", err)
	}

	config := &adk.Config{
		AgentLoader: services.NewSingleAgentLoader(agent),
	}

	l := full.NewLauncher()
	err = l.Execute(ctx, config, os.Args[1:])
	if err != nil {
		log.Fatalf("Error while executing agent: %v", err)
	}
}

type SumArgs struct {
	A int `json:"a"` // an integer to sum
	B int `json:"b"` // another integer to sum
}
type SumResult struct {
	Sum int `json:"sum"` // the sum of two integers
}

func makeSummer() (tool.Tool, error) {
	handler := func(ctx tool.Context, input SumArgs) SumResult {
		return SumResult{Sum: input.A + input.B}
	}
	return functiontool.New(functiontool.Config{
		Name:        "sum",
		Description: "sums two integers",
	}, handler)
}