fix error on Mac when a custom map references a vanilla tilesheet that only exists under Content/Maps

docs/release-notes.md

@@ -5,6 +5,7 @@ These changes have not been released yet.
 * For players:
   * Improved performance.
   * SMAPI now prevents invalid items from breaking menus on hover.
+  * Fixed error when a custom map references certain vanilla tilesheets on MacOS.
   * Fixed Save Backup not pruning old backups if they're uncompressed.
   * Fixed issues when a farmhand reconnects before the game notices they're disconnected.
   * Fixed 'received message' logs shown in non-developer mode.

src/SMAPI/Framework/ModHelpers/ContentHelper.cs

@@ -316,7 +316,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
             // check relative to content folder
             {
-                foreach (string candidateKey in new[] { imageSource, $@"Maps\{imageSource}" })
+                foreach (string candidateKey in new[] { imageSource, Path.Combine("Maps", imageSource) })
                 {
                     string contentKey = candidateKey.EndsWith(".png")
                         ? candidateKey.Substring(0, candidateKey.Length - 4)