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.
return; 
          }
          
          const variable = context.getScope().set.get(callback.name);
          if (variable == null || variable.defs == null) {
            
            return; 
          }
          
          
          
          const def = variable.defs[0];
          if (!def || !def.node) {
            break; 
          }
          if (def.type !== 'Variable' && def.type !== 'FunctionName') {
            
            break; 
          }
          switch (def.node.type) {
            case 'FunctionDeclaration':
              
              visitFunctionWithDependencies(
                def.node,
                declaredDependenciesNode,
                reactiveHook,
                reactiveHookName,
                isEffect,