fix

DBHelper/DB.cs

@@ -162,6 +162,10 @@ namespace DBHelper
                         Console.WriteLine(msg);
                         throw new Exception(msg);
                     }
+                    finally
+                    {
+                        db.Dispose();
+                    }
                 }
             }
         }
@@ -218,8 +222,9 @@ namespace DBHelper
                 ctx.Dispose();
             }
             Contexts.Clear();
-            Contexts = null;
         }
+
+        
     }
 
     public class DB<TContext> where TContext : DbContext