Add attention section in README.md (#13)

README.md

@@ -124,3 +124,8 @@ func getUsers(ctx echo.Context) error {
 	}
 }
 ```
+
+### Attentions
+
+This library is only available for the kind of functions that the [Query callback](https://pkg.go.dev/gorm.io/[email protected]/callbacks#Query) is executed on.<br>
+That is, it cannot be used with [Row](https://pkg.go.dev/gorm.io/[email protected]#DB.Row) or [Scan](https://pkg.go.dev/gorm.io/[email protected]#DB.Scan) <br>

cursor.go

@@ -322,7 +322,9 @@ func getCursorStringFromColumns(value reflect.Value, columns ...string) pbc.Curs
 	args := make([]interface{}, len(columns))
 	for i, column := range columns {
 		argValue := value.FieldByName(column)
-		if argValue.CanInterface() {
+		if !argValue.IsValid() {
+			panic("`" + column + "` field is not exist in " + value.Type().Name() + ".")
+		} else if argValue.CanInterface() {
 			args[i] = argValue.Interface()
 		} else {
 			args[i] = nil