• Check if updating to the latest Preact version resolves the issue

Describe the bug
Throwing an error within a Preact application with preact/debug enabled always logs the following warning in the dev console:

Add @babel/plugin-transform-react-jsx-source to get a more detailed component stack. Note that you should not add it to production builds of your App for bundle size reasons.

Adding @babel/plugin-transform-react-jsx-source to my babel configuration does not stop the warning.

This warning is logged from here in component-stack.js.

Looking at that file, I don't believe that hasBabelPlugin is ever set to true before this warning gets logged. Unfortunately, I don't know how to detect the presence or absence of a babel plugin, otherwise I'd suggest a patch!

To Reproduce

(I could not reproduce this in the provided codesandbox environment, maybe because it's using react-error-overlay?)

Example repo: https://github.com/imidom/preact-warning-example

1. Create a new Preact application using the latest version of preact-cli: npx preact-cli create default lol2 (or following instructions for a webpack - babel build)
2. Put an undefined variable into <App /> like the following:

   ...
   <Header />
    {eekAnUndefinedVariableThatWillCauseAnException}
   <Router>
   ...
   

3. Run the application: npm run dev
4. Load the application with your dev console open
   

Expected behavior
The error is logged without an accompanying warning about a missing babel plugin if the babel plugin has, in fact, been loaded. Though I was not able to see any difference in output with the plugin installed or removed.