eslint-plugin-react-hooks version: 4.2.0

## Steps To Reproduce

I know it shouldn't exist in production code, but it's annoying while editing some files.

```js
import React, { useEffect } from "react";

const App = () => {
  useEffect(); // <-
  return null;
};

export default App;
```

Link to code example: https://codesandbox.io/s/polished-sun-3wr27?file=/src/App.js

## The current behavior

ESLint crashed.

```
TypeError: Cannot read property 'type' of undefined
```

## The expected behavior

ESLint doesn't crash but reports an error instead.
let value3;
          let value4;
          let asyncValue;
          useEffect(() => {
            if (value4) {
              value = {};
            }
            value2 = 100;
            value = 43;
            value4 = true;
            console.log(value2);
            console.log(value3);
            setTimeout(() => {
              asyncValue = 100;
            });
          }, []);
        }
      `,
      
      
      
      errors: [
        {
          message:
            
            `Assignments to the 'value2' variable from inside React Hook useEffect ` +
            `will be lost after each render. To preserve the value over time, ` +
            `store it in a useRef Hook and keep the mutable value in the '.current' property. ` +
            `Otherwise, you can move this variable directly inside useEffect.`,
          suggestions: undefined,