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 (key.endsWith('.current')) {
            badRef = key;
          }
        });
        if (badRef !== null) {
          extraWarning =
            ` Mutable values like '${badRef}' aren't valid dependencies ` +
            "because mutating them doesn't re-render the component.";
        } else if (externalDependencies.size > 0) {
          const dep = Array.from(externalDependencies)[0];
          
          
          if (!scope.set.has(dep)) {
            extraWarning =
              ` Outer scope values like '${dep}' aren't valid dependencies ` +
              `because mutating them doesn't re-render the component.`;
          }
        }
      }
      
      
      
      if (!extraWarning && missingDependencies.has('props')) {
        const propDep = dependencies.get('props');
        if (propDep == null) {
          return;
        }
        const refs = propDep.references;
        if (!Array.isArray(refs)) {
          return;