Fix crash on locked subtitle select/move

src/timeline2/model/timelinemodel.cpp

@@ -1158,7 +1158,7 @@ int TimelineModel::suggestSubtitleMove(int subId, int position, int cursorPositi
     Q_ASSERT(isSubTitle(subId));
     int currentPos = getSubtitlePosition(subId);
     int offset = 0;
-    if (currentPos == position) {
+    if (currentPos == position || m_subtitleModel->isLocked()) {
         return position;
     }
     int newPos = position;
@@ -2157,6 +2157,11 @@ bool TimelineModel::requestGroupMove(int itemId, int groupId, int delta_track, i
             sorted_subtitles.push_back({affectedItemId, m_allSubtitles.at(affectedItemId)});
         }
     }
+    
+    if (!sorted_subtitles.empty() && m_subtitleModel->isLocked()) {
+        // Group with a locked subtitle, abort
+        return false;
+    }
 
     // Sort clips first
     std::sort(sorted_clips.begin(), sorted_clips.end(), [delta_pos](const std::pair<int, int> &clipId1, const std::pair<int, int> &clipId2) {