code: @CheckReturnValue
      public GuildManager setSystemChannel(TextChannel systemChannel)
      {
          Checks.check(systemChannel == null || systemChannel.getGuild().equals(getGuild()), "Channel must be from the same guild");
          this.systemChannel = systemChannel == null ? null : systemChannel.getId();
          set |= SYSTEM_CHANNEL;
          return this;
      }
docs: Sets the system {@link net.dv8tion.jda.core.entities.TextChannel TextChannel} of this {@link net.dv8tion.jda.core.entities.Guild Guild}.
  
  @param  systemChannel
  The new system channel for this {@link net.dv8tion.jda.core.entities.Guild Guild}
  or {@code null} to reset
  
  @throws IllegalArgumentException
  If the provided channel is not from this guild
  
  @return GuildManager for chaining convenience