Don't throw when parent channel cannot be retrieved

Closes #1108

DiscordChatExporter.Core/Discord/Data/Channel.cs

@@ -20,8 +20,9 @@ public partial record Channel(
     Snowflake? LastMessageId) : IHasId
 {
     // Used for visual backwards-compatibility with old exports, where
-    // channels without a parent (i.e. mostly DM channels) had a fallback
-    // category created for them.
+    // channels without a parent (i.e. mostly DM channels) or channels
+    // with an inaccessible parent (i.e. inside private categories) had
+    // a fallback category created for them.
     public string Category => Parent?.Name ?? Kind switch
     {
         ChannelKind.GuildCategory => "Category",

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -368,11 +368,21 @@ public class DiscordClient
             .GetNonWhiteSpaceStringOrNull()?
             .Pipe(Snowflake.Parse);
 
-        var parent = parentId is not null
-            ? await GetChannelAsync(parentId.Value, cancellationToken)
-            : null;
+        try
+        {
+            var parent = parentId is not null
+                ? await GetChannelAsync(parentId.Value, cancellationToken)
+                : null;
 
-        return Channel.Parse(response, parent);
+            return Channel.Parse(response, parent);
+        }
+        // It's possible for the parent channel to be inaccessible, despite the
+        // child channel being accessible.
+        // https://github.com/Tyrrrz/DiscordChatExporter/issues/1108
+        catch (DiscordChatExporterException)
+        {
+            return Channel.Parse(response);
+        }
     }
 
     private async ValueTask<Message?> TryGetLastMessageAsync(