When trying to use the render_template and render_template_string functions outside of the context of an application, a silly None error occurs that does not tell the user that this function only works when there is a flask application. I just fixed it.
I decided not to create a problem report about this, because this seems to be a fairly simple and small matter. Plus it's just like #4333 , which I've already fixed. Sorry, if this is still necessary, I will immediately create this message so that everything goes "by the rules".

Checklist:

• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.