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 (constantExpressionType != null) {
          return [ref, constantExpressionType];
        }
      }
      
      if (
        node.type === 'FunctionName' &&
        node.node.type === 'FunctionDeclaration'
      ) {
        return [ref, 'function'];
      }
      
      if (node.type === 'ClassName' && node.node.type === 'ClassDeclaration') {
        return [ref, 'class'];
      }
      return null;
    })
    .filter(Boolean);
  function isUsedOutsideOfHook(ref) {
    let foundWriteExpr = false;
    for (let i = 0; i < ref.references.length; i++) {
      const reference = ref.references[i];
      if (reference.writeExpr) {
        if (foundWriteExpr) {
          
          return true;
        } else {
          
          foundWriteExpr = true;
          continue;