In ArcGIS Web AppBuilder, the Basemap Gallery widget is set to custom basemaps including an extra basemap taken from the associated web map.

Workaround

This error does not occur when using the 'Always synchronize with the Basemap Gallery settings of the organization' setting in the configuration window. This can be edited in the organization settings under the Map tab and clicking Basemap Gallery. However, this update would be applied to the whole organization and might not be viable for many organizations.

Steps to Reproduce