下载远程webdav书籍时检查保存路径是否设置

app/src/main/java/io/legado/app/model/localBook/LocalBook.kt

@@ -308,6 +308,8 @@ object LocalBook {
         val webDavUrl = localBook.getRemoteUrl()
         if (webDavUrl.isNullOrBlank()) return null
         try {
+            AppConfig.defaultBookTreeUri
+                ?: throw NoStackTraceException("没有设置书籍保存位置!")
             val uri = AppWebDav.authorization?.let {
                 val webdav = WebDav(webDavUrl, it)
                 runBlocking {