fix open group message

libsession/src/main/java/org/session/libsession/messaging/opengroups/OpenGroupAPI.kt

@@ -143,7 +143,7 @@ object OpenGroupAPI: DotNetAPI() {
                         val dateAsString = message["created_at"] as String
                         val serverTimestamp = format.parse(dateAsString).time
                         // Verify the message
-                        val groupMessage = OpenGroupMessage(serverID, publicKey, displayName, body, timestamp, openGroupMessageType, quote, attachments, profilePicture, signature, serverTimestamp)
+                        val groupMessage = OpenGroupMessage(serverID, publicKey, displayName, body, timestamp, openGroupMessageType, quote, attachments.toMutableList(), profilePicture, signature, serverTimestamp)
                         if (groupMessage.hasValidSignature()) groupMessage else null
                     } catch (exception: Exception) {
                         Log.d("Loki", "Couldn't parse message for open group with ID: $channel on server: $server from: ${JsonUtil.toJson(message)}. Exception: ${exception.message}")

libsession/src/main/java/org/session/libsession/messaging/opengroups/OpenGroupMessage.kt

@@ -56,7 +56,7 @@ public data class OpenGroupMessage(
                         Attachment.Kind.LinkPreview,
                         server,
                         attachment.id,
-                        attachment.contentType,
+                        attachment.contentType!!,
                         attachment.size.get(),
                         attachment.fileName.get(),
                         0,
@@ -75,7 +75,7 @@ public data class OpenGroupMessage(
                         Attachment.Kind.Attachment,
                         server,
                         attachment.id,
-                        attachment.contentType,
+                        attachment.contentType!!,
                         attachment.size.get(),
                         attachment.fileName.get(),
                         0,