Check for null return when getting project url

[dkwon17]

Since document.querySelector can return null (doc), this small PR checks for undefined/null before calling getAttributes

To test this PR:

1. Build and sideload the extension: https://github.com/redhat-developer/try-in-dev-spaces-browser-extension#building-and-running-locally
2. When creating a new workspace with the injected button on GitHub, the correct project should be cloned in the workspace