simpler nullcheck

OwnChar/Manager/OwnCharManager.cs

@@ -9,7 +9,8 @@ namespace OwnChar.Manager;
 public class OwnCharManager
 {
     // User
-    public bool IsLoggedIn => CurrentUser != null;
+    [MemberNotNullWhen(true, nameof(CurrentUser), nameof(DataManager))]
+    public bool IsLoggedIn => CurrentUser != null && DataManager != null;
     public UserAccount? CurrentUser { get; private set; }
 
     // Data Provider
@@ -30,7 +31,7 @@ public class OwnCharManager
     [MemberNotNull(nameof(CurrentUser), nameof(DataManager))]
     internal protected void CheckLogin()
     {
-        if (DataManager == null || CurrentUser == null || !IsLoggedIn)
+        if (!IsLoggedIn)
             throw new LoginException("You are already logged in!");
     }