nbgitpuller link generator: Repo name doesn't get properly populated for Binder links using a Custom URL for "Application to Open"

### Bug description

Hello! I've been playing around with the nbgitpuller link generator for one of my projects and I noticed that when I try to use a Custom URL to open my notebook in (in this case, a rendered Voila dashboard), the repo name doesn't get properly populated in the link, instead showing "undefined". I think this bug is restricted to just the Binder panel.

#### Expected behaviour

I expect the repo name to be properly populated in the nbgitpuller link, like in this example when I choose the Classic Notebook interface.


#### Actual behaviour

"undefined" is being populated as the name of the repo in the nbgitpuller link when generated using a Custom URL for Binder links


### How to reproduce

1. Go to https://jupyterhub.github.io/nbgitpuller/link.html
2. Click on the Binder panel
3. Fill in the form with some dummy values
4. Select "Custom URL" for "Application to Open" and fill in a dummy value
5. See error in the generated link

### Your personal set up

- Browser: Google Chrome
- Version(s): 95.0.4638.69 (Official Build) (x86_64)