bug: crash on node shutdown with the unreleased v1.3.1

[brilliant-lx]

System information

Geth version: v1.3.1
OS & Version: Windows/Linux/OSX
Commit hash : 4493ab8

Expected behaviour

node can shutdown without crash

Actual behaviour

sometimes it will crash, it is not that high probability, may not easy to reproduce.

Steps to reproduce the behaviour

kill pidof bsc to shutdown the node.

Backtrace

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x77cf48]

goroutine 3871672 [running]:
github.com/ethereum/go-ethereum/event.JoinSubscriptions.func1.2({0x0?, 0x0?})
        github.com/ethereum/go-ethereum/event/multisub.go:34 +0x48
created by github.com/ethereum/go-ethereum/event.JoinSubscriptions.func1
        github.com/ethereum/go-ethereum/event/multisub.go:32 +0xc7