PANICS when I export genesis file at a specific height

[RevelationOfTuring]

System info: [Include Ethermint commit, operating system name, and other relevant details]

commit：4dfe396b (v0.3.1)
operating system：macOS Catalina 10.15.6
go version：go1.15.2 darwin/amd64

Steps to reproduce:

1. start a single node blockchain
2. start a rest-server with ethermintcli rest-server --laddr tcp://localhost:8545 --unlock-key xxx --chain-id ethermint-8 --trace
3. deploy an erc20 contract with Remix and Metamask
4. stop the node and export genesis file with ethermintd export --for-zero-height --height=70 (node was stopped at height 71)
5. panics with the log output:

panic: app.baseKey expected to be nil; duplicate init?

goroutine 1 [running]:
github.com/cosmos/cosmos-sdk/baseapp.(*BaseApp).initFromMainStore(0xc000e23400, 0xc000f7c0f0, 0x0, 0x0)
	/Users/oker/go/pkg/mod/github.com/cosmos/[email protected]/baseapp/baseapp.go:318 +0x2a0
github.com/cosmos/cosmos-sdk/baseapp.(*BaseApp).LoadVersion(0xc000e23400, 0x46, 0xc000f7c0f0, 0x4, 0xc000ddee58)
	/Users/oker/go/pkg/mod/github.com/cosmos/[email protected]/baseapp/baseapp.go:296 +0x80
github.com/cosmos/ethermint/app.(*EthermintApp).LoadHeight(...)
	/Users/oker/go/src/github.com/cosmos/ethermint/app/ethermint.go:350
main.exportAppStateAndTMValidators(0x54f6b60, 0xc0001af380, 0x550eea0, 0xc000010238, 0x0, 0x0, 0x46, 0xc0001af301, 0x5fa0030, 0x0, ...)
	/Users/oker/go/src/github.com/cosmos/ethermint/cmd/ethermintd/main.go:118 +0x1bc
github.com/cosmos/cosmos-sdk/server.ExportCmd.func1(0xc001310b00, 0xc000705500, 0x0, 0x2, 0x0, 0x0)
	/Users/oker/go/pkg/mod/github.com/cosmos/[email protected]/server/export.go:65 +0x53d
github.com/spf13/cobra.(*Command).execute(0xc001310b00, 0xc0007054e0, 0x2, 0x2, 0xc001310b00, 0xc0007054e0)
	/Users/oker/go/pkg/mod/github.com/spf13/[email protected]/command.go:850 +0x453
github.com/spf13/cobra.(*Command).ExecuteC(0xc0001ce2c0, 0x5102837, 0xc000e2fe60, 0x4185662)
	/Users/oker/go/pkg/mod/github.com/spf13/[email protected]/command.go:958 +0x349
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/oker/go/pkg/mod/github.com/spf13/[email protected]/command.go:895
github.com/tendermint/tendermint/libs/cli.Executor.Execute(0xc0001ce2c0, 0x5292470, 0x50e2a3c, 0x10)
	/Users/oker/go/pkg/mod/github.com/tendermint/[email protected]/libs/cli/setup.go:89 +0x3c
main.main()
	/Users/oker/go/src/github.com/cosmos/ethermint/cmd/ethermintd/main.go:91 +0x5b3

Expected behavior: [What you expected to happen]

Genesis file output with contract bytecode that deployed successfully

Actual behavior: [What actually happened]

Additional info: [Include gist of relevant config, logs, etc.]

[zhongqiuwood]

It hit panic even if no contract deployed