multiple runtime bugfixes for 1.5.5+65

lib/api/marianumcloud/talk/share_files_to_chat.dart

@@ -1,10 +1,35 @@
+import 'package:nextcloud/nextcloud.dart';
+
 import '../files_sharing/file_sharing_api.dart';
 import '../files_sharing/file_sharing_api_params.dart';
+import '../webdav/webdav_api.dart';
 
 /// WebDAV folder under which Talk-shared files are uploaded before being
 /// linked into a chat.
 const String talkShareFolder = 'MarianumMobile';
 
+Future<void>? _shareFolderReady;
+
+/// Creates [talkShareFolder] if it is missing, at most once per session — the
+/// folder is permanent, so every later upload would just pay a round trip to
+/// be told it already exists (WebDAV answers MKCOL on an existing collection
+/// with 405, which is the normal case here and must not surface as an error).
+Future<void> ensureTalkShareFolder() =>
+    _shareFolderReady ??= _createTalkShareFolder();
+
+Future<void> _createTalkShareFolder() async {
+  try {
+    final webdav = await WebdavApi.webdav;
+    await webdav.mkcol(PathUri.parse('/$talkShareFolder'));
+  } on DynamiteApiException catch (e) {
+    // Anything but "already exists" leaves the folder unconfirmed, so the next
+    // upload has to try again.
+    if (e.statusCode != 405) _shareFolderReady = null;
+  } catch (_) {
+    _shareFolderReady = null;
+  }
+}
+
 /// Posts each already-uploaded WebDAV path as a Talk share (ShareType 10) to
 /// the given conversation token. Calls run concurrently — the server accepts
 /// parallel posts and the picker UI is blocked anyway, so we shouldn't pay