GroupsManagerPlugin broken

event = InboundExceptionEvent{error=io.netty.handler.codec.DecoderException: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `org.drasyl.identity.ProofOfWork` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (io.netty.buffer.ByteBufInputStream); line: 1, column: 101] (through reference chain: org.drasyl.node.plugin.groups.client.message.GroupJoinMessage["proofOfWork"])}