fix an NRE I caused in movie code. The old code was wrong, this one is better

src/BizHawk.Client.Common/movie/MovieSession.cs

@@ -267,6 +267,8 @@ namespace BizHawk.Client.Common
 			{
 				Movie.StartNewPlayback();
 			}
+
+			MultiTrack.Restart(emulator.ControllerDefinition.PlayerCount);
 		}
 
 		public void ToggleMultitrack()
@@ -310,6 +312,8 @@ namespace BizHawk.Client.Common
 
 				message += "stopped.";
 
+				MultiTrack.Restart(1);
+
 				var result = Movie.Stop(saveChanges);
 				if (result)
 				{
@@ -319,7 +323,6 @@ namespace BizHawk.Client.Common
 				Output(message);
 				ReadOnly = true;
 
-				MultiTrack.Restart(Movie.Emulator.ControllerDefinition.PlayerCount);
 				_modeChangedCallback();
 			}
 

src/BizHawk.Client.Common/movie/bk2/Bk2Movie.InputLog.cs

@@ -39,7 +39,7 @@ namespace BizHawk.Client.Common
 			int? stateFrame = null;
 
 			// We are in record mode so replace the movie log with the one from the savestate
-			if (!Global.MovieSession.MultiTrack.IsActive) // TODO: this is run before being attached
+			if (!Session.MultiTrack.IsActive)
 			{
 				if (Global.Config.Movies.EnableBackupMovies && MakeBackup && Log.Count != 0)
 				{