Async invoke fixes and ButtonGrid update

- removing unused imports
- adding null check for async handler invokes
- fixing PageCount calculation on ButtonGrid

TelegramBotBase/Form/GroupForm.cs

@@ -5,7 +5,6 @@ using System.Text;
 using System.Threading.Tasks;
 using TelegramBotBase.Args;
 using TelegramBotBase.Base;
-using static TelegramBotBase.Base.Async;
 
 namespace TelegramBotBase.Form
 {

TelegramBotBase/Form/SplitterForm.cs

@@ -4,7 +4,7 @@ using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using TelegramBotBase.Base;
-using static TelegramBotBase.Base.Async;
+
 
 namespace TelegramBotBase.Form
 {