Panic on using map[string]interface{}

I am trying to use this library for evaluation of user provided expressions. Which means that there are no defined structs for objects. Instead it is a parsed JSON, (which simply outputs `map[string]interface{}`). However, this library seems unable to handle such use case? Or am I using it wrong way?

Example code:

```go
func main() {
userjson := `{"id": 123}`

filejson := `{"owner": 123}`

var User map[string]interface{}
var File map[string]interface{}

err := fastjson.Unmarshal([]byte(userjson), &User)
if err != nil {
panic(err)
}

err = fastjson.Unmarshal([]byte(filejson), &File)
if err != nil {
panic(err)
}

type Env map[string]interface{}

e := map[string]interface{}{
"file": File,
"user": User,
}

expr := "file.owner == user.id"

eval, err := bexpr.CreateEvaluatorForType(expr, nil, (*map[string]interface{})(nil))
if err != nil {
panic(err)
}

result, err := eval.Evaluate(e)
if err != nil {
panic(err)
}

fmt.Println(result)
}
```