fix: fix transaction hash nil pointer error

This commit ensures 'invalid memory address or nil pointer dereference'
is not thrown when trying to retrieve the transaction hash inside the
rewardservice.

eth/rewardservice.go

@@ -3,6 +3,7 @@ package eth
 import (
 	"context"
 	"fmt"
+	"strings"
 	"sync"
 
 	"github.com/ethereum/go-ethereum/core/types"
@@ -59,8 +60,8 @@ func (s *RewardService) Start(ctx context.Context) error {
 				if err != nil {
 					glog.Errorf("Error trying to call reward err=%q", err)
 					var txHash string
-					if tx != nil {
-						txHash = tx.Hash().String()
+					if tx != nil && !strings.Contains(err.Error(), "context deadline exceeded") {
+						txHash = tx.Hash().Hex()
 					}
 					if monitor.Enabled {
 						monitor.RewardCallError(ctx, err.Error(), s.tw.LastInitializedRound(), txHash)