[WebChannel] Input object does not work in release build

Steps to reproduce the issue.

1. Run the attached example.
2. Make a release build of the project and run

Expected result:

The html document loads and the text field is visible and you can enter text in it.

Actual result:

The html document loads, but the text field in not visible.

The example works with Qt 5.12.1 if you make a debug build of the project. Also, if you use Qt 5.10.1 or older the release build works as well so this looks like a regression.