Fix null branch UUID parsing

Author: YoshiRulz

src/BizHawk.Client.EmuHawk/tools/Lua/Libraries/TAStudioLuaLibrary.cs

@@ -468,7 +468,7 @@ public void LoadBranch(int index)
 		{
 			if (Engaged())
 			{
-				var marker = MarkerListForBranch(branchID).Get(frame);
+				var marker = MarkerListForBranch(branchID)?.Get(frame);
 				if (marker != null)
 				{
 					return marker.Message;
@@ -479,13 +479,10 @@ public void LoadBranch(int index)
 		}
 
 		/// <remarks>assumes a TAStudio project is loaded</remarks>
-		private TasMovieMarkerList MarkerListForBranch(string/*?*/ branchID)
-		{
-			var found = Guid.TryParseExact(branchID, format: "D", out var parsed)
-				? Tastudio.CurrentTasMovie.Branches.FirstOrDefault(branch => branch.Uuid == parsed)
-				: null;
-			return found?.Markers ?? Tastudio.CurrentTasMovie.Markers;
-		}
+		private TasMovieMarkerList/*?*/ MarkerListForBranch(string/*?*/ branchID)
+			=> Guid.TryParseExact(branchID, format: "D", out var parsed)
+				? Tastudio.CurrentTasMovie.Branches.FirstOrDefault(branch => branch.Uuid == parsed)?.Markers
+				: branchID is null ? Tastudio.CurrentTasMovie.Markers : null; // not a typo; null `branchID` indicates main log
 
 		[LuaMethodExample("""
 			local marker_label = tastudio.getmarker(tastudio.find_marker_on_or_before(100));
@@ -496,8 +493,8 @@ private TasMovieMarkerList MarkerListForBranch(string/*?*/ branchID)
 				+ " This may be the power-on marker at 0."
 				+ " If branchID is specified, searches the markers in that branch instead.")]
 		public int FindMarkerOnOrBefore(int frame, string/*?*/ branchID = null)
-			=> Engaged()
-				? MarkerListForBranch(branchID).PreviousOrCurrent(frame).Frame
+			=> Engaged() && MarkerListForBranch(branchID) is TasMovieMarkerList markers
+				? markers.PreviousOrCurrent(frame).Frame
 				: default;
 
 		[LuaMethodExample("""
@@ -508,8 +505,8 @@ public int FindMarkerOnOrBefore(int frame, string/*?*/ branchID = null)
 			description: "Returns a list of all the frames which have markers on them."
 				+ " If branchID is specified, instead returns the frames which have markers in that branch.")]
 		public LuaTable GetFramesWithMarkers(string/*?*/ branchID = null)
-			=> Engaged()
-				? _th.EnumerateToLuaTable(MarkerListForBranch(branchID).Select(static m => m.Frame))
+			=> Engaged() && MarkerListForBranch(branchID) is TasMovieMarkerList markers
+				? _th.EnumerateToLuaTable(markers.Select(static m => m.Frame))
 				: _th.CreateTable();
 
 		[LuaMethodExample("tastudio.removemarker( 500 );")]