Fix issue when changing to repo with no CloneUrl

Can't create a pull request session from a repository with no origin.

src/GitHub.InlineReviews/Services/PullRequestSessionManager.cs

@@ -227,6 +227,13 @@ namespace GitHub.InlineReviews.Services
 
         async Task<PullRequestSession> GetSessionInternal(string owner, string name, int number)
         {
+            var cloneUrl = repository.CloneUrl;
+            if (cloneUrl == null)
+            {
+                // Can't create a session from a repository with no origin
+                return null;
+            }
+
             PullRequestSession session = null;
             WeakReference<PullRequestSession> weakSession;
             var key = Tuple.Create(owner.ToLowerInvariant(), number);
@@ -238,7 +245,7 @@ namespace GitHub.InlineReviews.Services
 
             if (session == null)
             {
-                var address = HostAddress.Create(repository.CloneUrl);
+                var address = HostAddress.Create(cloneUrl);
                 var pullRequest = await sessionService.ReadPullRequestDetail(address, owner, name, number);
 
                 session = new PullRequestSession(