Can't open closed pull requests in Gitpod

[jankeromnes]

Describe the bug
Trying to open a closed pull request in Gitpod gives:

We have trouble parsing your context
Error: Cannot read property 'repository' of null

To Reproduce

1. Try to open a closed pull request in Gitpod, e.g. https://gitpod.io/#https://github.com/gitpod-io/gitpod-docs/pull/80

Expected behavior
It would be nice if we could open closed pull requests in Gitpod. Or, if that's not supported, maybe we should say that in a more explicit error message?

[AlexTugarev]

Same as #4?