The problem described happens for multiple controls. For the sake of simplicity, I'll stick with the DropDownList.

As documented here, the DropDownList's "fillMode" option supports "null" as value. However, the corresponding TS typings file does not allow null. The same is true for the "rounded" and "size" options as you can see in the snippet below that was copied from "index.d.ts".

interface DropDownListOptions {
        fillMode?: string | undefined;
        rounded?: string | undefined;
        size?: string | undefined;
    }

Note: parts of the code were omitted for brevity's sake.

A (temporary) workaround is to trick the typing system by casting manually:

fillMode: null as unknown as string | undefined