Show an appropriate hint if the room is inaccessible

resources/qml/TimelineView.qml

@@ -252,7 +252,7 @@ Item {
             Layout.rightMargin: Nheko.paddingLarge
 
             TextArea {
-                text: TimelineManager.escapeEmoji(preview.roomTopic)
+                text: preview.roomName != "" ? TimelineManager.escapeEmoji(preview.roomTopic) : qsTr("This room is possibly inaccessible. If this room is private, you should remove it from the child list of this space.")
                 wrapMode: TextEdit.WordWrap
                 textFormat: TextEdit.RichText
                 readOnly: true

src/timeline/RoomlistModel.cpp

@@ -222,7 +222,7 @@ RoomlistModel::data(const QModelIndex &index, int role) const
             case Roles::RoomName:
                 return tr("No preview available");
             case Roles::LastMessage:
-                return QString();
+                return tr("This room is possibly inaccessible");
             case Roles::Time:
                 return QString();
             case Roles::Timestamp: