优化

app/src/main/java/io/legado/app/utils/DocumentExtensions.kt

@@ -100,10 +100,10 @@ object DocumentUtils {
             return listFiles(uri.path!!, filter)
         }
         val docList = arrayListOf<FileDoc>()
-        var cursor: Cursor? = null
-        try {
         val childrenUri = DocumentsContract
             .buildChildDocumentsUriUsingTree(uri, DocumentsContract.getDocumentId(uri))
+        var cursor: Cursor? = null
+        try {
             cursor = appCtx.contentResolver.query(
                 childrenUri, arrayOf(
                     DocumentsContract.Document.COLUMN_DOCUMENT_ID,