Adding manual destination type throws TypeError: Cannot add property displayUrl, object is not extensible

[alanwthom]

Which version of Skybrud Redirects are you using? (Please write the exact version, example: 4.0.8)

15.0.0

Which Umbraco version are you using? (Please write the exact version, example: 10.1.0)

15.4.4

Bug description

Destination urls are not being not being saved from the Update Link modal when choosing a manual url.

Steps to reproduce:

1. From the redirects dashboard, click "Add redirect"
2. Specify original url (e.g. "/foo")
3. Click "Add" for destination url
4. Select manual and specify a source (e.g. "/bar")
5. Click "Update"

Destination url is not saved and the following error is thrown in the console:

Destination.js?v=783…11604865c8089b0:137 Uncaught (in promise) TypeError: Cannot add property displayUrl, object is not extensible
    at fromExternal (Destination.js?v=783…04865c8089b0:137:21)
    at Destination.js?v=783…04865c8089b0:229:34

[alanwthom]

I believe I've been able to trace this to the fromExternal() method in Destination.js. This looks to be inconsistent with both the fromContent() and fromMedia() methods, so I've attempted to resolve this and have opened a PR in respect to this. I think this will affect both the v15 and v16 implementations, but I have targeted v15 with the pull request. Please let me know if there's anything else you require.