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 foundCurrentAssignment = false;
          for (let i = 0; i < references.length; i++) {
            const {identifier} = references[i];
            const {parent} = identifier;
            if (
              parent != null &&
              
              
              parent.type === 'MemberExpression' &&
              !parent.computed &&
              parent.property.type === 'Identifier' &&
              parent.property.name === 'current' &&
              
              parent.parent.type === 'AssignmentExpression' &&
              parent.parent.left === parent
            ) {
              foundCurrentAssignment = true;
              break;
            }
          }
          
          if (foundCurrentAssignment) {
            return;
          }
          reportProblem({
            node: dependencyNode.parent.property,
            message:
              `The ref value '${dependency}.current' will likely have ` +
              `changed by the time this effect cleanup function runs. If ` +
              `this ref points to a node rendered by React, copy ` +