Syncing a project from github without a pxt.json file hangs the editor

Describe the bug
If your project doesn't have a pxt.json file, and you import it. It'll appear in the projects list, but clicking on it will cause the loading screen to show forever.

To Reproduce
Steps to reproduce the behavior:

1. Create a github repo that contains a main.blocks & main.ts file, but no pxt.json
2. Set up the arcade with you GitHub token
3. Try to import the code from the repo that you've created

Result
The loading screen remains forever.

Expected behavior
Either an option to add a pxt.json or an error message indicating that this project cannot be imported because it's missing a pxt.json file. I would probably only invest in the second as this seems like a relatively small scenario.

Reproduce the import using a GitHub repository containing main.blocks and main.ts but no pxt.json, after setting up the Arcade GitHub token. Trace the project import and editor-loading path to identify where the missing file is handled. Done means the import reports that pxt.json is missing instead of leaving the loading screen indefinitely.