Hi @OliverPannell do you happen to be on Windows 11 or have a folder named "tests" in your custom widgets directory? One of our developers is experiencing the same issue (Windows 11) but not myself (Windows 10).
@OliverPannell - similar issue we are dealing with: https://community.esri.com/t5/arcgis-experience-builder-questions/exb-1-9-dev-is-broken-strange-erro...