Avoid crash when trying to log nil group.Desc

Change-Id: I0c7bafa98e406ba4a0f3b61c6554bd167151835a
diff --git a/internal/pkg/core/openolt_flowmgr.go b/internal/pkg/core/openolt_flowmgr.go
index 3522ebd..b8875e7 100644
--- a/internal/pkg/core/openolt_flowmgr.go
+++ b/internal/pkg/core/openolt_flowmgr.go
@@ -2033,7 +2033,7 @@
 func (f *OpenOltFlowMgr) ModifyGroup(ctx context.Context, group *ofp.OfpGroupEntry) error {
 	logger.Infow("modify-group", log.Fields{"group": group})
 	if group == nil || group.Desc == nil {
-		return olterrors.NewErrInvalidValue(log.Fields{"group": group, "groupDesc": group.Desc}, nil)
+		return olterrors.NewErrInvalidValue(log.Fields{"group": group}, nil)
 	}
 
 	newGroup := f.buildGroup(group.Desc.GroupId, group.Desc.Buckets)