Hopefully block edge cases where global `GameInfo` is uninitialised

`Game == null` conditions in `MainForm` ctor looked unreachable, so I changed
them to `Game.IsNullInstance()` which is what I assume was intended, and added
an assert to `RomLoader` in case a bug is introduced later

src/BizHawk.Client.Common/RomLoader.cs

@@ -703,6 +703,8 @@ namespace BizHawk.Client.Common
 
 					return false;
 				}
+
+				_ = game!; // shouldn't be null if `nextEmulator` isn't? just in case
 			}
 			catch (Exception ex)
 			{

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

@@ -198,19 +198,12 @@ namespace BizHawk.Client.Common
 				movie.SyncSettingsJson = ConfigService.SaveWithType(settable.GetSyncSettings());
 			}
 
-			if (game.IsNullInstance())
-			{
-				movie.GameName = "NULL";
-			}
-			else
-			{
 			movie.GameName = game.FilesystemSafeName();
 			movie.Hash = game.Hash;
 			if (game.FirmwareHash != null)
 			{
 				movie.FirmwareHash = game.FirmwareHash;
 			}
-			}
 
 			if (emulator.HasBoardInfo())
 			{

src/BizHawk.Client.EmuHawk/MainForm.Events.cs

@@ -427,6 +427,7 @@ namespace BizHawk.Client.EmuHawk
 
 		private void RecordMovieMenuItem_Click(object sender, EventArgs e)
 		{
+			if (Game.IsNullInstance()) return;
 			if (!Emulator.Attributes().Released)
 			{
 				var result = this.ModalMessageBox2(

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -548,7 +548,7 @@ namespace BizHawk.Client.EmuHawk
 				var ioa = OpenAdvancedSerializer.ParseWithLegacy(_argParser.cmdRom);
 				if (ioa is OpenAdvanced_OpenRom oaor) ioa = new OpenAdvanced_OpenRom { Path = oaor.Path.MakeAbsolute() }; // fixes #3224; should this be done for all the IOpenAdvanced types? --yoshi
 				LoadRom(ioa.SimplePath, new LoadRomArgs { OpenAdvanced = ioa });
-				if (Game == null)
+				if (Game.IsNullInstance())
 				{
 					ShowMessageBox(owner: null, $"Failed to load {_argParser.cmdRom} specified on commandline");
 				}
@@ -567,7 +567,7 @@ namespace BizHawk.Client.EmuHawk
 			if (_argParser.cmdMovie != null)
 			{
 				_suppressSyncSettingsWarning = true; // We don't want to be nagged if we are attempting to automate
-				if (Game == null)
+				if (Game.IsNullInstance())
 				{
 					OpenRom();
 				}
@@ -3318,6 +3318,8 @@ namespace BizHawk.Client.EmuHawk
 		{
 			if (_currAviWriter != null) return;
 
+			if (Game.IsNullInstance()) throw new InvalidOperationException("how is an A/V recording starting with no game loaded? please report this including as much detail as possible");
+
 			// select IVideoWriter to use
 			IVideoWriter aw;
 
@@ -3431,19 +3433,12 @@ namespace BizHawk.Client.EmuHawk
 					}
 					else
 					{
-						using var sfd = new SaveFileDialog();
-						if (Game != null)
+						using SaveFileDialog sfd = new()
 						{
-							sfd.FileName = $"{Game.FilesystemSafeName()}.{ext}"; // don't use Path.ChangeExtension, it might wreck game names with dots in them
-							sfd.InitialDirectory = Config.PathEntries.AvAbsolutePath();
-						}
-						else
-						{
-							sfd.FileName = "NULL";
-							sfd.InitialDirectory = Config.PathEntries.AvAbsolutePath();
-						}
-
-						sfd.Filter = new FilesystemFilterSet(new FilesystemFilter(ext, new[] { ext })).ToString();
+							FileName = $"{Game.FilesystemSafeName()}.{ext}",
+							Filter = new FilesystemFilterSet(new FilesystemFilter(ext, new[] { ext })).ToString(),
+							InitialDirectory = Config.PathEntries.AvAbsolutePath(),
+						};
 
 						if (this.ShowDialogWithTempMute(sfd) == DialogResult.Cancel)
 						{

src/BizHawk.Client.EmuHawk/movie/RecordMovie.cs

@@ -30,6 +30,8 @@ namespace BizHawk.Client.EmuHawk
 			IMovieSession movieSession,
 			FirmwareManager firmwareManager)
 		{
+			if (game.IsNullInstance()) throw new InvalidOperationException("how is the traditional Record dialog open with no game loaded? please report this including as much detail as possible");
+
 			_mainForm = mainForm;
 			_config = config;
 			_game = game;

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.cs

@@ -583,6 +583,8 @@ namespace BizHawk.Client.EmuHawk
 				return;
 			}
 
+			if (Game.IsNullInstance()) throw new InvalidOperationException("how is TAStudio open with no game loaded? please report this including as much detail as possible");
+
 			var filename = DefaultTasProjName(); // TODO don't do this, take over any mainform actions that can crash without a filename
 			var tasMovie = (ITasMovie)MovieSession.Get(filename);
 			tasMovie.BindMarkersToInput = Settings.BindMarkersToInput;