For items with a custom URL it's impossible to upload files to a new bundle

[olli-gold]

Describe the bug
If an item has a custom URL, it's impossible to upload files to a new bundle via administration. It's failing, because the REST API call with the custom URL (https://dspace-instance/server/api/core/items/CUSTOMURL) cannot be responded correctly - it works only with the UUID.

For existing bundles it's still possible to upload new files with the upload button at the bundle; it's not working for the generic "Upload" button above the existing bundles.

To Reproduce
Steps to reproduce the behavior:

1. Create (or choose) an item with a custom URL
2. Go to the administrative Edit mode for the item
3. Choose the bitstreams tab
4. Click the upload button
5. Type a bundle name (there won't be a choice menu listing existing bundles!)
6. Click "Create bundle"

Nothing will happen, in the console you will see error messages.

Expected behavior
I would expect to see the upload option to put a new file into the bundle.

Related work
No related work I am aware of.