* Electron version: 1.6.15
* Operating system: Linux

### Expected behavior

Open a `BrowserWindow` with passed options

### Actual behavior

  - Usually a Segmentation fault.
  - Sometimes `[12843:0100/000000.043847:FATAL:zygote_linux.cc(479)] Failed to synchronise with parent zygote process`
  - Sometimes `[12819:0100/000000.719122:FATAL:zygote_linux.cc(509)] Check failed: len > 0 (0 vs. 0)`

### How to reproduce

I'm still working on making a smaller reproduction case. The issue goes away with small tweaks.

1. Starting from https://github.com/daurnimator/fengari-electron-example
2. Modify main.lua to contain:
```lua
local js = require "js"
local electron = require "electron"

electron.app:on("ready", function()
	local win = js.new(electron.BrowserWindow, js.createproxy({
		backgroundColor = '#2e2c29';
	}, "object"))
	win:loadURL("https://fengari.io")
end)
```

atom/browser/api/atom_api_window.cc
if (parent_window_.IsEmpty())
    return v8::Null(isolate());
  else
    return v8::Local<v8::Value>::New(isolate(), parent_window_);
}
std::vector<v8::Local<v8::Object>> Window::GetChildWindows() const {
  return child_windows_.Values(isolate());
}
v8::Local<v8::Value> Window::GetBrowserView() const {
  if (browser_view_.IsEmpty()) {
    return v8::Null(isolate());
  }
  return v8::Local<v8::Value>::New(isolate(), browser_view_);
}
void Window::SetBrowserView(v8::Local<v8::Value> value) {
  ResetBrowserView();
  mate::Handle<BrowserView> browser_view;
  if (value->IsNull()) {
    window_->SetBrowserView(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &browser_view)) {
    window_->SetBrowserView(browser_view->view());
    browser_view->web_contents()->SetOwnerWindow(window_.get());
    browser_view_.Reset(isolate(), value);
  }
}
void Window::ResetBrowserView() {