Fix latent NRE in Reopen seek and remove dead local

The deferred seek lambda in the Reopen flow re-read the SelectedSubtitle
property instead of the null-checked local snapshot. Since the post runs
on a later UI tick, closing the file or clearing the selection in that
window would make SelectedSubtitle null and throw an NRE on the UI
thread. Use the captured local `s`, which is already null-checked.

Also drop the now-unused colorTextTooLong local in
SubtitleTextInfoHelper.Populate (CPS coloring switched to
ColorCharactersPerSecond).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: niksedk

src/ui/Features/Main/MainViewModel.cs

@@ -1834,7 +1834,7 @@ private void CommandFileReopen(RecentFile recentFile)
                 if (vp != null && s != null)
                 {
                     vp.Position = s.StartTime.TotalSeconds;
-                    Dispatcher.UIThread.Post(() => { vp.Position = SelectedSubtitle.StartTime.TotalSeconds; });
+                    Dispatcher.UIThread.Post(() => { vp.Position = s.StartTime.TotalSeconds; });
                     CenterAudioVisualizerOnCurrentVideoPosition();
                 }
             }

src/ui/Logic/SubtitleTextInfoHelper.cs

@@ -38,7 +38,6 @@ internal static void Populate(
         out IBrush totalBackground)
     {
         var info = PopulateLineLengthsAndTotal(text, panel);
-        var colorTextTooLong = Se.Settings.General.ColorTextTooLong;
         var maxCps = Se.Settings.General.SubtitleMaximumCharactersPerSeconds;
 
         var cps = GetCharactersPerSecond(text, item.StartTime, item.EndTime);