* 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
if (IsDevToolsOpened())
    CloseDevTools();
  else
    OpenDevTools(nullptr);
}
void WebContents::InspectElement(int x, int y) {
  if (type_ == REMOTE)
    return;
  if (!enable_devtools_)
    return;
  if (!managed_web_contents()->GetDevToolsWebContents())
    OpenDevTools(nullptr);
  managed_web_contents()->InspectElement(x, y);
}
void WebContents::InspectServiceWorker() {
  if (type_ == REMOTE)
    return;
  if (!enable_devtools_)
    return;
  for (const auto& agent_host : content::DevToolsAgentHost::GetOrCreateAll()) {
    if (agent_host->GetType() ==
        content::DevToolsAgentHost::kTypeServiceWorker) {
      OpenDevTools(nullptr);
      managed_web_contents()->AttachTo(agent_host);
      break;
    }
  }
}
void WebContents::HasServiceWorker(