Bug report

When a custom class attribute is set via the columns.attributes option, the TreeList renders an additional class that is undefined.

Reproduction of the problem

1. Initialize a TreeList.

2. Set a custom class via the attributes option:

3. Inspect the relevant "td" element

Dojo sample for reproduction:

https://dojo.telerik.com/IqApoGOz

Expected/desired behavior

The TreeList should render only the specified classes.

Environment

* **Kendo UI version:** 2019.3.1023
* **jQuery version:** 1.12.4
* **Browser:** [all]