Closes #1543 (#1944)

* Start working on nullable checks

help me

* Update GlobalConfig.cs

* Finish initial fixup round

* nullability code review

ArchiSteamFarm.CustomPlugins.ExamplePlugin/ExamplePlugin.cs

@@ -55,7 +55,7 @@ namespace ArchiSteamFarm.CustomPlugins.ExamplePlugin {
 		// Thanks to that, you can extend default ASF config with your own stuff, then parse it here in order to customize your plugin during runtime
 		// Keep in mind that, as noted in the interface, additionalConfigProperties can be null if no custom, unrecognized properties are found by ASF, you should handle that case appropriately
 		// In addition to that, this method also guarantees that all plugins were already OnLoaded(), which allows cross-plugins-communication to be possible
-		public void OnASFInit(IReadOnlyDictionary<string, JToken> additionalConfigProperties = null) {
+		public void OnASFInit(IReadOnlyDictionary<string, JToken>? additionalConfigProperties = null) {
 			if (additionalConfigProperties == null) {
 				return;
 			}
@@ -80,14 +80,14 @@ namespace ArchiSteamFarm.CustomPlugins.ExamplePlugin {
 		// Since ASF already had to do initial parsing in order to determine that the command is unknown, args[] are splitted using standard ASF delimiters
 		// If by any chance you want to handle message in its raw format, you also have it available, although for usual ASF pattern you can most likely stick with args[] exclusively. The message has CommandPrefix already stripped for your convenience
 		// If you do not recognize the command, just return null/empty and allow ASF to gracefully return "unknown command" to user on usual basis
-		public async Task<string> OnBotCommand(Bot bot, ulong steamID, string message, string[] args) {
+		public async Task<string?> OnBotCommand(Bot bot, ulong steamID, string message, string[] args) {
 			// In comparison with OnBotMessage(), we're using asynchronous CatAPI call here, so we declare our method as async and return the message as usual
 			// Notice how we handle access here as well, it'll work only for FamilySharing+
 			switch (args[0].ToUpperInvariant()) {
 				case "CAT" when bot.HasPermission(steamID, BotConfig.EPermission.FamilySharing):
 					// Notice how we can decide whether to use bot's AWH WebBrowser or ASF's one. For Steam-related requests, AWH's one should always be used, for third-party requests like those it doesn't really matter
 					// Still, it makes sense to pass AWH's one, so in case you get some errors or alike, you know from which bot instance they come from. It's similar to using Bot's ArchiLogger compared to ASF's one
-					string randomCatURL = await CatAPI.GetRandomCatURL(bot.ArchiWebHandler.WebBrowser).ConfigureAwait(false);
+					string? randomCatURL = await CatAPI.GetRandomCatURL(bot.ArchiWebHandler.WebBrowser).ConfigureAwait(false);
 
 					return !string.IsNullOrEmpty(randomCatURL) ? randomCatURL : "God damn it, we're out of cats, care to notify my master? Thanks!";
 				default:
@@ -126,7 +126,7 @@ namespace ArchiSteamFarm.CustomPlugins.ExamplePlugin {
 		// Keep in mind that, as noted in the interface, additionalConfigProperties can be null if no custom, unrecognized properties are found by ASF, you should handle that case appropriately
 		// Also keep in mind that this function can be called multiple times, e.g. when user edits his bot configs during runtime
 		// Take a look at OnASFInit() for example parsing code
-		public async void OnBotInitModules(Bot bot, IReadOnlyDictionary<string, JToken> additionalConfigProperties = null) {
+		public async void OnBotInitModules(Bot bot, IReadOnlyDictionary<string, JToken>? additionalConfigProperties = null) {
 			// ASF marked this message as synchronous, in case we have async code to execute, we can just use async void return
 			// For example, we'll ensure that every bot starts paused regardless of Paused property, in order to do this, we'll just call Pause here in InitModules()
 			// Thanks to the fact that this method is called with each bot config reload, we'll ensure that our bot stays paused even if it'd get unpaused otherwise
@@ -143,19 +143,22 @@ namespace ArchiSteamFarm.CustomPlugins.ExamplePlugin {
 		// Keep in mind that there is no guarantee what is the actual access of steamID, so you should do the appropriate access checking yourself
 		// You can use either ASF's default functions for that, or implement your own logic as you please
 		// If you do not intend to return any response to user, just return null/empty and ASF will proceed with the silence as usual
-		public Task<string> OnBotMessage(Bot bot, ulong steamID, string message) {
+		public Task<string?> OnBotMessage(Bot bot, ulong steamID, string message) {
 			// Normally ASF will expect from you async-capable responses, such as Task<string>. This allows you to make your code fully asynchronous which is a core foundation on which ASF is built upon
 			// Since in this method we're not doing any async stuff, instead of defining this method as async (pointless), we just need to wrap our responses in Task.FromResult<>()
+			if (Bot.BotsReadOnly == null) {
+				throw new ArgumentNullException(nameof(Bot.BotsReadOnly));
+			}
 
 			// As a starter, we can for example ignore messages sent from our own bots, since otherwise they can run into a possible infinite loop of answering themselves
 			if (Bot.BotsReadOnly.Values.Any(existingBot => existingBot.SteamID == steamID)) {
-				return Task.FromResult<string>(null);
+				return Task.FromResult<string?>(null);
 			}
 
 			// If this message doesn't come from one of our bots, we can reply to the user in some pre-defined way
 			bot.ArchiLogger.LogGenericTrace("Hey boss, we got some unknown message here!");
 
-			return Task.FromResult("I didn't get that, did you mean to use a command?");
+			return Task.FromResult((string?) "I didn't get that, did you mean to use a command?");
 		}
 
 		// This method is called when bot receives a trade offer that ASF isn't willing to accept (ignored and rejected trades)