[escher] TableView::cellAtLocation should return nullptr if the cell is

invisible and therefore no view displays it.

Fix bug: add 4 functions, in the values table one cell is wrong as soon
as you edit an x value

escher/src/selectable_table_view.cpp

@@ -158,6 +158,7 @@ void SelectableTableView::unhighlightSelectedCell() {
   if (selectedColumn() >= 0 && selectedColumn() < dataSource()->numberOfColumns() &&
       selectedRow() >= 0 && selectedRow() < dataSource()->numberOfRows()) {
     HighlightCell * previousCell = cellAtLocation(selectedColumn(), selectedRow());
+    assert(previousCell);
     previousCell->setHighlighted(false);
   }
 }