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.
`Move it inside the useEffect callback. Alternatively, ` +
            `wrap the definition of 'handleNext' in its own useCallback() Hook.`,
          
          
          suggestions: undefined,
        },
      ],
    },
    {
      
      
      
      
      
      code: normalizeIndent`
        function MyComponent(props) {
          let [, setState] = useState();
          const handleNext = (value) => {
            setState(value);
          };
          useEffect(() => {
            return Store.subscribe(handleNext);
          }, [handleNext]);
          return <div onClick={handleNext} />;
        }
      `,
      errors: [
        {
          message:
            `The 'handleNext' function makes the dependencies of ` +