Fix Cell add exception in SelectBetweenMarkers

- closes #3971

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.MenuItems.cs

@@ -378,6 +378,7 @@ private void SelectBetweenMarkersMenuItem_Click(object sender, EventArgs e)
 				int prev = prevMarker?.Frame ?? 0;
 				int next = nextMarker?.Frame ?? CurrentTasMovie.InputLogLength;
 
+				TasView.DeselectAll();
 				for (int i = prev; i < next; i++)
 				{
 					TasView.SelectRow(i, true);