Bug: When creating new Site, the Language doesn't load the Default language.

[WeiweiCaiAcpt]

When creating new site, if I only fill the Title and then click Save button, it will show 400 Bad Request in Developer tool.

When checking the Network from Developer tool, it shows:
{errors: {,…}, type: "https://tools.ietf.org/html/rfc7231#section-6.5.1",…} errors: {,…} **languageId: ["Error converting value {null} to type 'System.Guid'. Path 'languageId', line 1, position 76."]** 0: "Error converting value {null} to type 'System.Guid'. Path 'languageId', line 1, position 76." status: 400 title: "One or more validation errors occurred." traceId: "|a48ec097-4a128de4aba7e734." type: "https://tools.ietf.org/html/rfc7231#section-6.5.1"

But when I went back to Add site, I found I cannot select Language because it is not loaded anything, even I tried add a language.

If I Edit an existing Site, the Language can load successful. This issue only occurs when creating new site.

Reproduce steps:

1. Login to the app.
2. Click Add Site button.
3. Fill "Test" for Title.
4. Select Language, but nothing can be selected.