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### Preflight Checklist
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* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
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* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * v8.0.0-beta.2
* **Operating System:**
  * macOS 10.14.5
* **Last Known Working Electron version:**
  * v6.1.X 

### Expected Behavior
No warnings

### Actual Behavior
The warning

```
[28455:1113/114404.211707:ERROR:CONSOLE(3361)] "Uncaught TypeError: sender.emit is not iterable (cannot read property Symbol(Symbol.iterator))", source: electron/js2c/sandbox_bundle.js (3361)
```

is printed in the console constantly.

### To Reproduce
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```sh
$ git clone $YOUR_URL -b $BRANCH
$ npm install
$ npm start || electron .
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### Screenshots

![image](https://user-images.githubusercontent.com/380914/68798485-5cd39b00-060b-11ea-80d9-1f62cbb97e52.png)


shell/browser/api/atom_api_web_contents.cc
std::string footer;
  options.Get("footer", &footer);
  if (!(header.empty() && footer.empty())) {
    settings.SetBoolKey(printing::kSettingHeaderFooterEnabled, true);
    settings.SetStringKey(printing::kSettingHeaderFooterTitle, header);
    settings.SetStringKey(printing::kSettingHeaderFooterURL, footer);
  } else {
    settings.SetBoolKey(printing::kSettingHeaderFooterEnabled, false);
  }
  // We don't want to allow the user to enable these settings
  // but we need to set them or a CHECK is hit.
  settings.SetIntKey(printing::kSettingPrinterType, printing::kLocalPrinter);
  settings.SetBoolKey(printing::kSettingShouldPrintSelectionOnly, false);
  settings.SetBoolKey(printing::kSettingRasterizePdf, false);
  // Set custom page ranges to print
  std::vector<gin_helper::Dictionary> page_ranges;
  if (options.Get("pageRanges", &page_ranges)) {
    base::Value page_range_list(base::Value::Type::LIST);
    for (auto& range : page_ranges) {
      int from, to;
      if (range.Get("from", &from) && range.Get("to", &to)) {
        base::Value range(base::Value::Type::DICTIONARY);
        range.SetIntKey(printing::kSettingPageRangeFrom, from);
        range.SetIntKey(printing::kSettingPageRangeTo, to);
        page_range_list.Append(std::move(range));
      } else {
        continue;
      }
    }
    if (page_range_list.GetList().size() > 0)