Hi Dave, the _sourceMenuButtonNode is a private property on the Search Widget, when you are debugging, you can inspect the properties of the SearchWidget and find it. This is the dropdown button which lets you select which search source you want to use.
Is the error preventing webpack from packing the .tsx files? I'm using Visual Studio 2019, I get the same warning, but it does not prevent the files from compiling. I am also using ExB 1.2, however, i built this widget in 1.1.
If you are not concerned with letting people chose a search source, then you can remove the code related to the search source drop down.
Cheers