Add a null check for the DataGridView instance before calling the…

… `NotifyMSAAClient` method.

Fixes #12752

## Root Cause

- The issue occurs because the `NotifyMSAAClient` method is called
without checking if the `DataGridView` instance is `null`. This leads to
a potential `NullReferenceException`.

## Proposed changes

- Add a `null` check for the `DataGridView` instance before calling the
`NotifyMSAAClient` method.

## Customer Impact

- Prevents runtime crashes when the `DataGridView` is `null`.

## Regression?

- No

## Risk

- Minimal

## Screenshots

### Before

### After

## Test methodology

- Manual testing

## Test environment(s)

- `10.0.100-alpha.1.25064.3`

src/System.Windows.Forms/src/System/Windows/Forms/Controls/DataGridView/DataGridViewCheckBoxCell.cs

@@ -839,7 +839,10 @@ protected override void OnKeyUp(KeyEventArgs e, int rowIndex)
                 e.Handled = true;
             }
 
-            NotifyMSAAClient(ColumnIndex, rowIndex);
+            if (DataGridView is not null)
+            {
+                NotifyMSAAClient(ColumnIndex, rowIndex);
+            }
         }
     }