Invalid POST JSON body fails silently

Q&A (please complete the following information)

• OS: Ubuntu 18
• Browser: chrome
• Version: 80.0.3987.87 (Official Build) (64-bit)
• Method of installation: npm
• Swagger-UI version: 3.25.0
• Swagger/OpenAPI version: [e.g. Swagger 2.0, OpenAPI 3.0]

Describe the bug you're encountering

An invalid JSON body doesn't display an error message or state. If you enter invalid JSON and click execute, nothing seems to happen. This can be reproduced on this endpoint that accepts JSON.

To reproduce...

Steps to reproduce the behavior:

1. Go to https://petstore.swagger.io/#/pet/addPet
2. Click on try it out
3. change the body to any invalid JSON string
4. Attempt to execute the request

Expected behavior

An error message or some state change that makes it clear that the input is invalid.

Screenshots

Additional context or thoughts

I wasn't able to track down any open issues for this, but I did find an old one that may be related: https://github.com/swagger-api/swagger-ui/issues/5648