Fix NullPointerException

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/BotImpl.kt

@@ -28,7 +28,7 @@ abstract class BotImpl<N : BotNetworkHandler> constructor(
     final override val account: BotAccount = account
     override val uin: Long
         get() = account.id
-    final override val logger: MiraiLogger = configuration.logger ?: DefaultLogger("Bot($uin)").also { configuration.logger = it }
+    final override val logger: MiraiLogger by lazy { configuration.logger ?: DefaultLogger("Bot($uin)").also { configuration.logger = it } }
 
     init {
         @Suppress("LeakingThis")