Minor fix

* Fix selection error when use delete after to delete the last tas unit
* Fix sel_rect display error
* Add some documents
* Fix layout becoming confusing when secondly open file.

BallanceTASEditor/Core/TASFile.cs

@@ -38,7 +38,8 @@ namespace BallanceTASEditor.Core {
         }
 
         public long GetPointerIndex() {
-            if (mPointer == null) throw new Exception("Data is not ready");
+            // return invalid data to prevent error
+            if (mPointer == null) return -1;//throw new Exception("Data is not ready");
             return mPointerIndex;
         }