Steps to reproduce:
Open some subtitles together with a movie (File->Open with selected Video to open) to see a crash.
When subtitles are opened first and video by Video->Open later it also crashes. Only video opened is loaded correctly.
Unfortunately it's not always reproducible, not for every file.

Stack trace:
[szpak@mszpak build]$ mono ./gnome-subtitles.exe
Got 21 languages.
Got video duration: 00:43:06.8800000
Got video info: width=512, height=384, frame_rate=25
Got video duration: 01:59:25.7600000
Got video info: width=432, height=240, frame_rate=25
Detected encodings: windows-1252
Trying the encoding Windows-1252
Trying to autodetect the subtitle format.
[*] opened /some/path/movie/movie
movie.txt with encoding I18N.West.CP1252 and format Micro DVD
Detected encodings: windows-1252
Trying the encoding Windows-1252
Trying to autodetect the subtitle format.
[*] opened /some/path/movie/movie.txt with encoding I18N.West.CP1252 and format Micro DVD
Got video duration: 01:59:25.7600000

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
at GnomeSubtitles.Ui.VideoPreview.Player.get_FrameRate () <0x0001c>
at GnomeSubtitles.Ui.VideoPreview.VideoPosition.UpdateLengthLabel (SubLib.Core.Domain.TimingMode,System.TimeSpan) <0x000bb>
at GnomeSubtitles.Ui.VideoPreview.VideoPosition.SetLength (System.TimeSpan) <0x00042>
at GnomeSubtitles.Ui.VideoPreview.VideoPosition.OnPlayerFoundDuration (System.TimeSpan) <0x00020>
at GnomeSubtitles.Ui.VideoPreview.Player.OnPlaybinFoundTag (GStreamer.TagEventArgs) <0x00105>
at GStreamer.Playbin.onTag (intptr) <0x00054>
at (wrapper native-to-managed) GStreamer.Playbin.onTag (intptr) <0x00048>
at (wrapper managed-to-native) Gtk.Application.gtk_main () <0x00004>
at Gtk.Application.Run () <0x0000a>
at GnomeSubtitles.Execution.ExecutionContext.RunApplication () <0x00027>
at GnomeSubtitles.Core.Base.Run (GnomeSubtitles.Execution.ExecutionContext) <0x00034>
at GnomeSubtitles.Execution.Executable.Main (string[]) <0x0006f>


Other information:
GIT version a0894b4..3ca0361.
Works fine with gnome-subtitles-0.8-7.fc11.
Not tested with 0.9.