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System.NullReferenceException was unhandled
  Message=Object reference not set to an instance of an object.
  Source=System.Windows.Forms
  StackTrace:
       at System.Windows.Forms.DataGridViewCheckBoxCell.NotifyMASSClient(Point position)
       at System.Windows.Forms.DataGridViewCheckBoxCell.OnKeyUp(KeyEventArgs e, Int32 rowIndex)
       at System.Windows.Forms.DataGridView.OnKeyUp(KeyEventArgs e)
       at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
       at System.Windows.Forms.DataGridView.ProcessKeyEventArgs(Message& m)
       at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.DataGridView.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.RunDialog(Form form)
       at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
       at System.Windows.Forms.Form.ShowDialog()
       at BendBiegeFormView.Editing.Editor.DoEditForm() in C:\*\BendBiegeFormView\Editing\Editor.cs:line 223
       at BendBiegeFormView.Editing.Editor.New() in C:\*\BendBiegeFormView\Editing\Editor.cs:line 109
       at TestForm.Form1.button1_Click(Object sender, EventArgs e) in C:\*\TestForm\Form1.cs:line 38
       at System.Windows.Forms.Control.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ButtonBase.WndProc(Message& m)
       at System.Windows.Forms.Button.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.Run(Form mainForm)
       at TestForm.Program.Main() in C:\*\TestForm\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException:                    

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        private void NotifyMASSClient(Point position)
        { 
            Debug.Assert((position.X >= 0) && (position.X < this.DataGridView.Columns.Count));
            Debug.Assert((position.Y >= 0) && (position.Y < this.DataGridView.Rows.Count));


            int visibleRowIndex = this.DataGridView.Rows.GetRowCount(DataGridViewElementStates.Visible, 0, position.Y); //<--- Hier Exception!
            int visibleColumnIndex = this.DataGridView.Columns.ColumnIndexToActualDisplayIndex(position.X, DataGridViewElementStates.Visible);
 
            int topHeaderRowIncrement = this.DataGridView.ColumnHeadersVisible ? 1 : 0; 
            int rowHeaderIncrement = this.DataGridView.RowHeadersVisible ? 1 : 0;
 
            int objectID = visibleRowIndex + topHeaderRowIncrement  // + 1 because the top header row acc obj is at index 0
                                           + 1;                     // + 1 because objectID's need to be positive and non-zero

            int childID = visibleColumnIndex + rowHeaderIncrement;  // + 1 because the column header cell is at index 0 in top header row acc obj 
                                                                    //     same thing for the row header cell in the data grid view row acc obj
 
            (this.DataGridView.AccessibilityObject as Control.ControlAccessibleObject).NotifyClients(AccessibleEvents.StateChange, objectID, childID); 
        }