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.
if (child.isUsed) {
        
        missingPaths.add(path);
        
        
        return;
      }
      scanTreeRecursively(
        child,
        missingPaths,
        satisfyingPaths,
        childKey => path + '.' + childKey,
      );
    });
  }
  
  const suggestedDependencies = [];
  const unnecessaryDependencies = new Set();
  const duplicateDependencies = new Set();
  declaredDependencies.forEach(({key}) => {
    
    if (satisfyingDependencies.has(key)) {
      if (suggestedDependencies.indexOf(key) === -1) {
        
        suggestedDependencies.push(key);
      } else {
        
        duplicateDependencies.add(key);
      }
    } else {