When I resize the window I get so now and then the above issue. Please add a check whether "window" is undefined or not. It is at line number 70738 in de kendo.all.js of version 2018.2.620
The fontSize tool gets focus after text selection.
* Open and run https://dojo.telerik.com/oRuLayir demo.
* Select /highlight Line4, choose 16 in the fontSize dropdown.
* Put the cursor at the end of Line6, select Line4, Line5 and Line6 using the mouse. Note that the keyboard focus has shifted to the fontSize dropdown.