* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
content::RenderFrameHost* frame_host) {
  bool handled = true;
  auto relay = NativeWindowRelay::FromWebContents(web_contents());
  if (!relay)
    return false;
  IPC_BEGIN_MESSAGE_MAP_WITH_PARAM(NativeWindow, message, frame_host)
    IPC_MESSAGE_FORWARD(AtomAutofillFrameHostMsg_ShowPopup,
      relay->window.get(), NativeWindow::ShowAutofillPopup)
    IPC_MESSAGE_FORWARD(AtomAutofillFrameHostMsg_HidePopup,
      relay->window.get(), NativeWindow::HideAutofillPopup)
    IPC_MESSAGE_UNHANDLED(handled = false)
  IPC_END_MESSAGE_MAP()
  return handled;
}
void WebContents::WebContentsDestroyed() {