* Electron version: 2.0.0-beta.4
* Operating system: macOS 10.13.3

### Expected behavior

A reload of the window works.

### Actual behavior

A reload of the window does not work.

### How to reproduce

* run VSCode exploration ([download](https://az764295.vo.msecnd.net/exploration/6bc3274250cc949c7fb1d654cbb6e59c8ddad697/VSCode-darwin-exploration.zip))
* run "Reload Window" command
* sometimes I see the window stay blank after doing so and checking the console I see:

```
Uncaught (in promise) Error: Cannot get property 'id' on missing remote object 3
Error: Cannot get property 'id' on missing remote object 3
    at throwRPCError (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/rpc-server.js:143:17)
    at EventEmitter.<anonymous> (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/rpc-server.js:384:7)
    at emitThree (events.js:136:13)
    at EventEmitter.emit (events.js:217:7)
    at WebContents.<anonymous> (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/api/web-contents.js:295:13)
    at emitTwo (events.js:126:13)
    at WebContents.emit (events.js:214:7)
    at throwRPCError (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/rpc-server.js:143:17)
    at EventEmitter.<anonymous> (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/rpc-server.js:384:7)
    at emitThree (events.js:136:13)
    at EventEmitter.emit (events.js:217:7)
    at WebContents.<anonymous> (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/browser/api/web-contents.js:295:13)
    at emitTwo (events.js:126:13)
    at WebContents.emit (events.js:214:7)
    at Object.exception (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/renderer/api/remote.js:209:30)
    at metaToValue (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/renderer/api/remote.js:213:28)
    at Object.descriptor.get [as id] (/Applications/Visual Studio Code - Exploration.app/Contents/Resources/electron.asar/renderer/api/remote.js:132:16)
    at file:///Applications/Visual%20Studio%20Code%20-%20Exploration.app/Contents/Resources/app/out/vs/workbench/electron-browser/bootstrap/index.js:11:97
    at new Promise (<anonymous>)
    at file:///Applications/Visual%20Studio%20Code%20-%20Exploration.app/Contents/Resources/app/out/vs/workbench/electron-browser/bootstrap/index.js:10:838
```
atom/browser/atom_browser_client.cc
content::RenderFrameHost* rfh,
    content::BrowserContext* browser_context,
    content::SiteInstance* current_instance,
    const GURL& url,
    content::SiteInstance** new_instance) {
  if (g_suppress_renderer_process_restart) {
    g_suppress_renderer_process_restart = false;
    return;
  }
  if (!ShouldCreateNewSiteInstance(rfh, browser_context, current_instance, url))
    return;
  bool is_new_instance = true;
  scoped_refptr<content::SiteInstance> site_instance;
  
  std::string affinity;
  auto* web_contents = content::WebContents::FromRenderFrameHost(rfh);
  auto* web_preferences = web_contents ?
      WebContentsPreferences::FromWebContents(web_contents) : nullptr;
  if (web_preferences &&
      web_preferences->web_preferences()->GetString("affinity", &affinity) &&
      !affinity.empty()) {
    affinity = base::ToLowerASCII(affinity);
    auto iter = site_per_affinities.find(affinity);
    if (iter != site_per_affinities.end()) {