apply same fix to colorForCell for consistency

Author: ericlln

packages/iris-grid/src/IrisGridTreeTableModel.ts

@@ -213,8 +213,6 @@ class IrisGridTreeTableModel extends IrisGridTableModelTemplate<
       if (value != null) {
         const column = this.sourceColumn(x, y);
         const row = this.row(y);
-        assertNotNull(row);
-
         let columnTypeForFormatting = column.type;
 
         // For tree table leaf nodes, use the constituent type for formatting