* 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::NavigationHandle* navigation_handle) {
  bool is_main_frame = navigation_handle->IsInMainFrame();
  if (navigation_handle->HasCommitted() && !navigation_handle->IsErrorPage()) {
    auto url = navigation_handle->GetURL();
    bool is_in_page = navigation_handle->IsSameDocument();
    if (is_main_frame && !is_in_page) {
      Emit("did-navigate", url);
    } else if (is_in_page) {
      Emit("did-navigate-in-page", url, is_main_frame);
    }
  } else {
    auto url = navigation_handle->GetURL();
    int code = navigation_handle->GetNetErrorCode();
    auto description = net::ErrorToShortString(code);
    Emit("did-fail-provisional-load", code, description, url, is_main_frame);
    
    if (code != net::ERR_ABORTED)
      Emit("did-fail-load", code, description, url, is_main_frame);
  }
}
void WebContents::TitleWasSet(content::NavigationEntry* entry,