Hotfix cancelling unsigned 1.20.6 commands

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/chat/session/SessionCommandHandler.java

@@ -55,7 +55,7 @@ public class SessionCommandHandler implements CommandHandler<SessionPlayerComman
                   + "Contact your network administrator."));
         }
         // We seemingly can't actually do this if signed args exist, if not, we can probs keep stuff happy
-        if (player.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_19_3)) {
+        if (player.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_19_3) && packet.lastSeenMessages != null) {
           return CompletableFuture.completedFuture(new ChatAcknowledgementPacket(packet.lastSeenMessages.getOffset()));
         }
         return CompletableFuture.completedFuture(null);