Rasa Playground stuck on "Training..."

Does the Rasa Playground not display error messages if there’s an issue with the training data? It seems that it gets stuck and continuously displays “Training…” (with the rotating dots) in the browser window.

To test this, I introduced an error in the default “activate subscribe form” rule; namely, I changed “- action: newsletter_form” to “- action: subscribe”. Then I clicked the “Train” button. It’s been stuck on “Training…” for the past 20 minutes.

I realize the Rasa Playground is not the preferred environment for developing a virtual assistant, but my students are not able to install Rasa on their computers. If it’d be possible to display error messages in the “Training” window, that would be extremely helpful!