NPE when creating vertex with link via studio

OrientDB Version: orientdb-3.1.6

Java Version: 11.0.5

OS: mac os x (10.15.7, x86_64)

Expected behavior

I want to be able to create a new vertex that has a mandatory link property.

Actual behavior

NPE when saving the new vertex:

Internal server error:

java.lang.NullPointerException
        at com.orientechnologies.orient.core.record.impl.ODocument.validateField(ODocument.java:786)
        at com.orientechnologies.orient.core.record.impl.ODocument.validate(ODocument.java:2625)
        at com.orientechnologies.orient.core.db.document.ODatabaseDocumentAbstract.saveInternal(ODatabaseDocumentAbstract.java:1830)
        at com.orientechnologies.orient.core.db.document.ODatabaseDocumentAbstract.save(ODatabaseDocumentAbstract.java:1802)
        at com.orientechnologies.orient.core.db.document.ODatabaseDocumentAbstract.save(ODatabaseDocumentAbstract.java:135)
        at com.orientechnologies.orient.core.record.impl.ODocument.save(ODocument.java:2361)
        at com.orientechnologies.orient.core.record.impl.ODocument.save(ODocument.java:2350)
        at com.orientechnologies.orient.server.network.protocol.http.command.post.OServerCommandPostDocument.execute(OServerCommandPostDocument.java:54)
        at com.orientechnologies.orient.server.network.protocol.http.ONetworkProtocolHttpAbstract.service(ONetworkProtocolHttpAbstract.java:253)
        at com.orientechnologies.orient.server.network.protocol.http.ONetworkProtocolHttpAbstract.execute(ONetworkProtocolHttpAbstract.java:811)
        at com.orientechnologies.common.thread.OSoftThread.run(OSoftThread.java:67)

Steps to reproduce

1. I installed orientDB, edited orientdb.sh (properly set ORIENTDB_DIR und ORIENTDB_USER) and run it with start arg.
2. I opened studio at http://localhost:2480/ and logged in
3. I created a new vertex Country with some string properties
4. I created a new vertex Address with some string properties and a link property called Country pointing to County vertex and with min=1, max=1, mandatory=true and not null = true.
5. I created a record of Country with all properties filled and noted its RID
6. I created a record of Address with all properties filled and the Country property set to the before noted RID.

At 6. saving failed with the above exception.