Added more null checking in some error handling code in the legacy plugin loader

IPA.Loader/Loader/PluginManager.cs

@@ -241,7 +241,7 @@ namespace IPA.Loader
             catch (ReflectionTypeLoadException e)
             {
                 Logger.loader.Error($"Could not load the following types from {Path.GetFileName(file)}:");
-                Logger.loader.Error($"  {string.Join("\n  ", e.LoaderExceptions.Select(e1 => e1?.Message))}");
+                Logger.loader.Error($"  {string.Join("\n  ", e.LoaderExceptions?.Select(e1 => e1?.Message) ?? new string[0])}");
             }
             catch (Exception e)
             {