TopSQL is unavailable caused by panic

[zhongzc]

[2022/10/18 08:02:47.357 +00:00] [ERROR] [misc.go:21] ["panic in the recoverable goroutine"] [r="\"invalid memory address or nil pointer dereference\""] ["stack trace"="github.com/pingcap/ng-monitoring/utils.GoWithRecovery.func1\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/utils/misc.go:23\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:884\nruntime.panicmem\n\t/usr/local/go/src/runtime/panic.go:260\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_unix.go:835\ngithub.com/pingcap/ng-monitoring/config/pdvariable.(*variableLoader).loadGlobalConfigLoop\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/config/pdvariable/pdvariable.go:108\ngithub.com/pingcap/ng-monitoring/config/pdvariable.(*variableLoader).start\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/config/pdvariable/pdvariable.go:65\ngithub.com/pingcap/ng-monitoring/utils.GoWithRecovery\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/utils/misc.go:26"] [stack="github.com/pingcap/ng-monitoring/utils.GoWithRecovery.func1\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/utils/misc.go:21\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:884\nruntime.panicmem\n\t/usr/local/go/src/runtime/panic.go:260\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_unix.go:835\ngithub.com/pingcap/ng-monitoring/config/pdvariable.(*variableLoader).loadGlobalConfigLoop\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/config/pdvariable/pdvariable.go:108\ngithub.com/pingcap/ng-monitoring/config/pdvariable.(*variableLoader).start\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/config/pdvariable/pdvariable.go:65\ngithub.com/pingcap/ng-monitoring/utils.GoWithRecovery\n\t/home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/ng-monitoring/utils/misc.go:26"]