Panic error on config parsing when `update_message: true` in `slack_configs`

What did you do?

Hi team,
I'm trying to enable https://github.com/prometheus/alertmanager/pull/5007, but Alertmanager fails to parse config file whenever I specify update_message: true (see broken config and logs in attachement).

When update_message: false or I set api_url for each notifier, Alertmanager is able to parse the config.

# amconfig_good.yaml
global:
  slack_api_url: "https://slack.com/api/chat.postMessage"
route:
  receiver: foo
receivers:
  - name: foo
    slack_configs:
      - channel: bar-channel
        send_resolved: true
        update_message: true
        api_url: "https://slack.com/api/chat.postMessage"
      - channel: bar-channel
        send_resolved: true
        update_message: false

$ amtool check-config amconfig_good.yaml
Checking 'amconfig_good.yaml'  SUCCESS
Found:
 - global config
 - route
 - 0 inhibit rules
 - 1 receivers
 - 0 templates

What did you expect to see?

Validation should respect global.slack_api_url config or throw error in human-readable format

What did you see instead? Under which circumstances?

Fatal error during config unmarshaling

System information

Darwin 25.4.0 arm64

Alertmanager version

version: 0.32.0
branch: main
revision: bf98d8214337743a22f2eb7b35980f15ba745829

`amtool` doesn't report version correctly when installed with `go install github.com/prometheus/alertmanager/cmd/amtool@v0.32.0`

$ amtool --version
amtool, version  (branch: , revision: unknown)
  build user:
  build date:
  go version:       go1.26.2
  platform:         darwin/arm64
  tags:             unknown

Alertmanager configuration file

# amconfig_bad.yaml
global:
  slack_api_url: "https://slack.com/api/chat.postMessage"
route:
  receiver: foo
receivers:
  - name: foo
    slack_configs:
      - channel: bar-channel
        send_resolved: true
        update_message: true

Prometheus version

Not relevant

Prometheus configuration file

Not relevant

Logs

$ amtool check-config amconfig_bad.yaml
Checking 'amconfig.yaml'panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x104cb7018]

goroutine 1 [running]:
gopkg.in/yaml%2ev2.handleErr(0x10242137f9d8)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:249 +0x6c
panic({0x1055e1780?, 0x1057e39c0?})
	/opt/homebrew/Cellar/go/1.26.2/libexec/src/runtime/panic.go:860 +0x12c
gopkg.in/yaml%2ev2.handleErr(0x10242137ecd0)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:249 +0x6c
panic({0x1055e1780?, 0x1057e39c0?})
	/opt/homebrew/Cellar/go/1.26.2/libexec/src/runtime/panic.go:860 +0x12c
gopkg.in/yaml%2ev2.handleErr(0x10242137e5b0)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:249 +0x6c
panic({0x1055e1780?, 0x1057e39c0?})
	/opt/homebrew/Cellar/go/1.26.2/libexec/src/runtime/panic.go:860 +0x12c
github.com/prometheus/alertmanager/config.(*SlackConfig).UnmarshalYAML(0x102421452380, 0x107d94a70?)
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/config/notifiers.go:585 +0x188
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler(0x102421136de0, 0x10242142a7e0, {0x107d94a70, 0x102421452380})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:270 +0x90
gopkg.in/yaml%2ev2.(*decoder).prepare(0x102421136de0, 0x10242142a7e0, {0x1055e8640?, 0x1024210e2738?, 0x10242137e058?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:313 +0x25c
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a7e0, {0x1055e8640?, 0x1024210e2738?, 0x1024210f2d80?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:364 +0x180
gopkg.in/yaml%2ev2.(*decoder).sequence(0x102421136de0, 0x10242142a770, {0x10556b760?, 0x102421452230?, 0x10242137e198?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:609 +0x2a0
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a770, {0x10556b760?, 0x102421452230?, 0x19?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:374 +0x1d8
gopkg.in/yaml%2ev2.(*decoder).mappingStruct(0x102421136de0, 0x10242142a5b0, {0x105708960?, 0x1024214521c0?, 0x105554b00?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:767 +0x9e4
gopkg.in/yaml%2ev2.(*decoder).mapping(0x102421136de0?, 0x10242142a5b0?, {0x105708960?, 0x1024214521c0?, 0x10242137e548?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:626 +0x384
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a5b0, {0x105554b00?, 0x1024214521c0?, 0x10?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:372 +0x1f4
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler.func1({0x105554b00?, 0x1024214521c0?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:272 +0xc4
github.com/prometheus/alertmanager/config.(*Receiver).UnmarshalYAML(0x1024214521c0, 0x107d94a50?)
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/config/config.go:952 +0x34
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler(0x102421136de0, 0x10242142a5b0, {0x107d94a50, 0x1024214521c0})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:270 +0x90
gopkg.in/yaml%2ev2.(*decoder).prepare(0x102421136de0, 0x10242142a5b0, {0x1057084e0?, 0x1024214521c0?, 0x10242137e778?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:313 +0x25c
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a5b0, {0x1057084e0?, 0x1024214521c0?, 0x1024210f2d50?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:364 +0x180
gopkg.in/yaml%2ev2.(*decoder).sequence(0x102421136de0, 0x10242142a540, {0x10556bc20?, 0x1024214371f8?, 0x10242137e8b8?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:609 +0x2a0
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a540, {0x10556bc20?, 0x1024214371f8?, 0x10566ab60?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:374 +0x1d8
gopkg.in/yaml%2ev2.(*decoder).mappingStruct(0x102421136de0, 0x10242142a0e0, {0x1056de4c0?, 0x1024214371d0?, 0x105554a40?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:767 +0x9e4
gopkg.in/yaml%2ev2.(*decoder).mapping(0x102421136de0?, 0x10242142a0e0?, {0x1056de4c0?, 0x1024214371d0?, 0x10242137ec68?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:626 +0x384
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a0e0, {0x105554a40?, 0x1024214371d0?, 0x10242137ecb8?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:372 +0x1f4
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler.func1({0x105554a40?, 0x1024214371d0?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:272 +0xc4
github.com/prometheus/alertmanager/config.(*Config).UnmarshalYAML(0x1024214371d0, 0x107d949b0?)
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/config/config.go:289 +0x3c
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler(0x102421136de0, 0x10242142a0e0, {0x107d949b0, 0x1024214371d0})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:270 +0x90
gopkg.in/yaml%2ev2.(*decoder).prepare(0x102421136de0, 0x10242142a0e0, {0x1056e2c40?, 0x1024214371d0?, 0x10555a500?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:313 +0x25c
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a0e0, {0x1056e2c40?, 0x1024214371d0?, 0x10242137f978?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:364 +0x180
gopkg.in/yaml%2ev2.(*decoder).document(...)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:384
gopkg.in/yaml%2ev2.(*decoder).unmarshal(0x102421136de0, 0x10242142a070, {0x1056e2c40?, 0x1024214371d0?, 0x10242137f9f8?})
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/decode.go:360 +0x15c
gopkg.in/yaml%2ev2.unmarshal({0x1024214400e0, 0xdb, 0xe0}, {0x1055e83c0, 0x1024214371d0}, 0x1)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:148 +0x2ec
gopkg.in/yaml%2ev2.UnmarshalStrict(...)
	/Users/hzieba/go/pkg/mod/gopkg.in/yaml.v2@v2.4.0/yaml.go:89
github.com/prometheus/alertmanager/config.Load({0x102421440000, 0xdb})
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/config/config.go:120 +0x54
github.com/prometheus/alertmanager/config.LoadFile({0x16b7cb08c, 0xd})
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/config/config.go:146 +0x3c
github.com/prometheus/alertmanager/cli.CheckConfig({0x102421438290?, 0x10242137fb48?, 0x10242137fc68?})
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/cli/check_config.go:67 +0x188
github.com/prometheus/alertmanager/cli.(*checkConfigCmd).checkConfig(...)
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/cli/check_config.go:48
github.com/alecthomas/kingpin/v2.(*actionMixin).applyActions(...)
	/Users/hzieba/go/pkg/mod/github.com/alecthomas/kingpin/v2@v2.4.0/actions.go:28
github.com/alecthomas/kingpin/v2.(*Application).applyActions(0x102421428100?, 0x1024211a8750)
	/Users/hzieba/go/pkg/mod/github.com/alecthomas/kingpin/v2@v2.4.0/app.go:568 +0xe4
github.com/alecthomas/kingpin/v2.(*Application).execute(0x102421428100, 0x1024211a8750, {0x102421438260, 0x1, 0x1})
	/Users/hzieba/go/pkg/mod/github.com/alecthomas/kingpin/v2@v2.4.0/app.go:398 +0x50
github.com/alecthomas/kingpin/v2.(*Application).Parse(0x102421428100, {0x1024212b0280?, 0x1024212b0280?, 0x102421419260?})
	/Users/hzieba/go/pkg/mod/github.com/alecthomas/kingpin/v2@v2.4.0/app.go:230 +0xe8
github.com/prometheus/alertmanager/cli.Execute()
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/cli/root.go:184 +0xff8
main.main()
	/Users/hzieba/go/pkg/mod/github.com/prometheus/alertmanager@v0.32.0/cmd/amtool/main.go:19 +0x1c

Reproduce the failure with amtool check-config using the amconfig_bad.yaml example, then trace SlackConfig.UnmarshalYAML in config/notifiers.go and the loading path through config/config.go and cli/check_config.go. Done means the same configuration no longer panics and validation either honors global.slack_api_url or reports a human-readable error.