FIx more issues with challenges.

src/Aiursoft.ChessServer/Controllers/HomeController.cs

@@ -120,7 +120,8 @@ public class HomeController : Controller
         var challenge = _database.GetChallenge(id);
         if (challenge == null)
         {
-            return RedirectToAction(nameof(Index));
+            // Challenge not found.
+            return NotFound();
         }
         var model = new ChallengeViewModel
         {

src/Aiursoft.ChessServer/Data/InMemoryDatabase.cs

@@ -73,6 +73,11 @@ public class InMemoryDatabase : ISingletonDependency
     {
         lock (Challenges)
         {
+            if (Challenges.All(t => t.Value.Permission != ChallengePermission.Public))
+            {
+                return null;
+            }
+            
             return Challenges
                 .FirstOrDefault(t => t.Value.Permission == ChallengePermission.Public)
                 .Key;

src/Aiursoft.ChessServer/Views/Home/Challenge.cshtml

@@ -3,7 +3,7 @@
 
 <div class="jumbotron">
     <div class="container">
-        <h1 class="display-4">Waiting for joining...</h1>
+        <h1 class="display-4">Waiting for opponent joining...</h1>
         <p class="lead">Please share the link of this room to your friend!</p>
         @{
             var link = $"{Context.Request.Scheme}://{Context.Request.Host}/Home/{nameof(HomeController.Challenge)}/{Model.RoomId}";

src/Aiursoft.ChessServer/Views/Home/Index.cshtml

@@ -33,7 +33,7 @@
                                     <td>@item.Value.Message</td>
                                     <td>@item.Value.Creator.NickName</td>
                                     <td>
-                                        <button class="btn btn-sm btn-primary" data-room-id="@item.Key">Join</button>
+                                        <a class="btn btn-sm btn-primary" asp-controller="Home" asp-action="Challenge" asp-route-id="@item.Key">Join</a>
                                     </td>
                                 </tr>
                             }