This issue is caused by the header, button, or body properties in the sharedTheme object returned from the Portal to be null. Usually, it would return an object as:
{ header: {}, body, {}, button: {}, ... }, which the properties are not empty.
I will look into this, and in the meantime, please add a logo, or colors to the header, button, or body to force the sharedTheme to not be null.