There is an issue when comparing the checkboxColumn and treeColumn.

The checkboxColumn variable contains a list filtered by the condition showCheckbox = true. The treeColumn variable represents the first column of the column list.

Later, a strict comparison occurs between checkboxColumn (which is a list of objects) and treeColumn (which is always an object). Therefore, the comparison will always evaluate to false, leading to a false positive message: 'ShowCheckBox column should not be defined other than the tree column'.

https://github.com/syncfusion/ej2-javascript-ui-controls/blame/5914ec806e2ebe6b57fabb690d970e3f7a0cc9d6/controls/treegrid/src/treegrid/base/treegrid.ts#L2200