getIn always returns undefined

### Are you submitting a **bug report** or a **feature request**?
**bug report**

### What is the current behaviour?

The getIn function given with the tools provided to mutators always returns undefined.
I can't get it to work under any circonstances, complex or simple names, I've created a simpler version do demonstrate:

```
render() {
return (

{console.log(tools.getIn(state, name), state)}
}}
/>

);
}

renderForm = ({handleSubmit, submitting, form: { mutators: {testMutator}}}) => {
return (

...

{strings.precision_country_label}

testMutator('country')}>Test

...
)
}
```

The result of the log :


### What is the expected behavior?

It should return the value of the field that I request with the complexKey param.
I've also logged the state param, and the field does exist with its value

### Sandbox Link

### What's your environment?

"react-final-form": "^6.3.3",
"react-final-form-arrays": "^3.1.1",
"final-form": "^4.18.6",
"final-form-arrays": "^3.0.2",
"react": "^16.9.0",
"react-dom": "^16.9.0",

I've only tested with Chrome (78.0.3904.108) on Mac

### Other information

Thank you for your time, and this excellent package that I love!