Throws `IllegalStateException: This message event did not happen in a guild`

[risu729]

The bot throws IllegalStateException while the pack auto-generation.
It occurs because the settings check whether pack auto-generation is enabled.
To fix this, skip the check if the message is in DMs.

java.lang.IllegalStateException: This message event did not happen in a guild
at net.dv8tion.jda.api.events.message.GenericMessageEvent.getGuild(GenericMessageEvent.java:163)
at io.github.risu729.erutcurts.structure.StructureListener.onMessageReceived(StructureListener.java:46)
at net.dv8tion.jda.api.hooks.ListenerAdapter.onEvent(ListenerAdapter.java:425)
at net.dv8tion.jda.api.hooks.InterfacedEventManager.handle(InterfacedEventManager.java:96)
at net.dv8tion.jda.internal.hooks.EventManagerProxy.handleInternally(EventManagerProxy.java:88)
at net.dv8tion.jda.internal.hooks.EventManagerProxy.handle(EventManagerProxy.java:70)
at net.dv8tion.jda.internal.JDAImpl.handleEvent(JDAImpl.java:181)
at net.dv8tion.jda.internal.handle.MessageCreateHandler.handleInternally(MessageCreateHandler.java:136)
at net.dv8tion.jda.internal.handle.SocketHandler.handle(SocketHandler.java:39)
at net.dv8tion.jda.internal.requests.WebSocketClient.onDispatch(WebSocketClient.java:983)
at net.dv8tion.jda.internal.requests.WebSocketClient.onEvent(WebSocketClient.java:869)
at net.dv8tion.jda.internal.requests.WebSocketClient.handleEvent(WebSocketClient.java:847)
at net.dv8tion.jda.internal.requests.WebSocketClient.onBinaryMessage(WebSocketClient.java:1022)
at com.neovisionaries.ws.client.ListenerManager.callOnBinaryMessage(ListenerManager.java:385)
at com.neovisionaries.ws.client.ReadingThread.callOnBinaryMessage(ReadingThread.java:276)
at com.neovisionaries.ws.client.ReadingThread.handleBinaryFrame(ReadingThread.java:996)
at com.neovisionaries.ws.client.ReadingThread.handleFrame(ReadingThread.java:755)
at com.neovisionaries.ws.client.ReadingThread.main(ReadingThread.java:108)
at com.neovisionaries.ws.client.ReadingThread.runMain(ReadingThread.java:64)
at com.neovisionaries.ws.client.WebSocketThread.run(WebSocketThread.java:45)