Application error when custom button does not set `styleClass`

[bampakoa]

When a custom button does not have the styleClass property set, the application throws the following error:

core.mjs:6469 ERROR Error: NgClass can only toggle CSS classes expressed as strings, got undefined

The styleClass is an optional property of the TButton interface so we can assume that the consumer of the library may or may not use the styleClass in their custom buttons. To reproduce the error, use the documentation example and comment out the styleClass property in the custom button.