UnsupportedOperationException when building scenario with schematron

[wustudent]

The code of v1.5.0 contains a bug, which leads to UnsupportedOperationException when using the API to build scenario with schematron.

See #135 for detail.

[phax]

Part of Release v1.5.2 from https://github.com/itplr-kosit/validator/releases/tag/v1.5.2