simapp: use tmjson on InitChainer (cosmos#7514)

Co-authored-by: Alessio Treglia <alessio@tendermint.com>

app.go

@@ -9,6 +9,7 @@ import (
 	"github.com/gorilla/mux"
 	"github.com/rakyll/statik/fs"
 	abci "github.com/tendermint/tendermint/abci/types"
+	tmjson "github.com/tendermint/tendermint/libs/json"
 	"github.com/tendermint/tendermint/libs/log"
 	tmos "github.com/tendermint/tendermint/libs/os"
 	tmproto "github.com/tendermint/tendermint/proto/tendermint/types"
@@ -450,7 +451,9 @@ func (app *SimApp) EndBlocker(ctx sdk.Context, req abci.RequestEndBlock) abci.Re
 // InitChainer application update at chain initialization
 func (app *SimApp) InitChainer(ctx sdk.Context, req abci.RequestInitChain) abci.ResponseInitChain {
 	var genesisState GenesisState
-	app.cdc.MustUnmarshalJSON(req.AppStateBytes, &genesisState)
+	if err := tmjson.Unmarshal(req.AppStateBytes, &genesisState); err != nil {
+		panic(err)
+	}
 	return app.mm.InitGenesis(ctx, app.appCodec, genesisState)
 }