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.
`'${dependency}.current' to a variable inside the effect, and ` +
              `use that variable in the cleanup function.`,
          });
        },
      );
      
      
      const staleAssignments = new Set();
      function reportStaleAssignment(writeExpr, key) {
        if (staleAssignments.has(key)) {
          return;
        }
        staleAssignments.add(key);
        reportProblem({
          node: writeExpr,
          message:
            `Assignments to the '${key}' variable from inside React Hook ` +
            `${context.getSource(reactiveHook)} 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 ` +
            `${context.getSource(reactiveHook)}.`,
        });
      }
      
      const stableDependencies = new Set();
      dependencies.forEach(({isStable, references}, key) => {
        if (isStable) {
          stableDependencies.add(key);
        }