tiup playground scale-in panics

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

tiup playground nightly --db 0
tiup playground scale-out --db 1
tiup playground scale-in --pid xxxxx

2. What did you expect to see?

The TiDB node added by scale-out is stopped.

3. What did you see instead?

The scane-in command's output:

Error: Post "http://127.0.0.1:9527/command": EOF
Error: run `/home/tidb/.tiup/components/playground/v1.4.1/tiup-playground` (wd:/home/tidb/.tiup/data/SUj1ZcK) failed: exit status 1

tiup playground's output:

receive command: scale-in
2021-04-16 06:51:32.365126 I | http: panic serving 127.0.0.1:45502: runtime error: invalid memory address or nil pointer dereference
goroutine 193 [running]:
net/http.(*conn).serve.func1(0xc00040a000)
        net/http/server.go:1824 +0x153
panic(0x5618ed0b1f80, 0x5618ed7a27a0)
        runtime/panic.go:971 +0x499
github.com/pingcap/tiup/components/playground/instance.(*TiFlashInstance).Pid(0xc0001a7ad0, 0x16241)
        <autogenerated>:1 +0x34
main.(*Playground).handleScaleIn.func1(0x5618eca761b7, 0x7, 0x5618ed20b9d8, 0xc0001a7ad0, 0x0, 0x0)
        github.com/pingcap/tiup/components/playground/playground.go:251 +0x52
main.(*Playground).WalkInstances(0xc000168000, 0xc001415988, 0xc0001b4060, 0x1a)
        github.com/pingcap/tiup/components/playground/playground.go:574 +0x46e
main.(*Playground).handleScaleIn(0xc000168000, 0x5618ed1ed1f8, 0xc0003240e0, 0x16241, 0xc001415a08, 0x1)
        github.com/pingcap/tiup/components/playground/playground.go:250 +0xb4
main.(*Playground).handleCommand(0xc000168000, 0xc0001e2310, 0x5618ed1ed1f8, 0xc0003240e0, 0xc0003240e0, 0x0)
        github.com/pingcap/tiup/components/playground/playground.go:469 +0x177
main.(*Playground).commandHandler(0xc000168000, 0x5618ed2044c0, 0xc0003240e0, 0xc0008fc000)
        github.com/pingcap/tiup/components/playground/playground.go:503 +0x334
net/http.HandlerFunc.ServeHTTP(0xc0004e7190, 0x5618ed2044c0, 0xc0003240e0, 0xc0008fc000)
        net/http/server.go:2069 +0x46
net/http.(*ServeMux).ServeHTTP(0x5618ed7bfb40, 0x5618ed2044c0, 0xc0003240e0, 0xc0008fc000)
        net/http/server.go:2448 +0x1ad
net/http.serverHandler.ServeHTTP(0xc000325a40, 0x5618ed2044c0, 0xc0003240e0, 0xc0008fc000)
        net/http/server.go:2887 +0xa5
net/http.(*conn).serve(0xc00040a000, 0x5618ed2065f0, 0xc000188180)
        net/http/server.go:1952 +0x8cd
created by net/http.(*Server).Serve
        net/http/server.go:3013 +0x3ad

4. What version of TiUP are you using (tiup --version)?

Didn't understand what actual version it is.

~$ tiup --version
v1.3.2 tiup
Go Version: go1.13
Git Branch: release-1.3
GitHash: 2d88460
~$ tiup playground --version
Starting component `playground`: /home/tidb/.tiup/components/playground/v1.4.1/tiup-playground --version
tiup version 1.4.1 tiup
Go Version: go1.16.3
Git Ref: v1.4.1
GitHash: cd19b75b6418f627d121d43d4b1e41af673526cf

Start with components/playground/playground.go at handleScaleIn around lines 250-251 and inspect WalkInstances around line 574, using the reported TiFlashInstance.Pid nil-pointer stack trace. Reproduce with the listed playground scale-out and scale-in commands. Done means scale-in stops the added TiDB node without panicking or returning the HTTP EOF error.