fix error when a location doesn't have a map asset key

docs/release-notes.md

@@ -2,6 +2,7 @@
 ## 2.9.1
 * For players:
   * Fixed crash in SMAPI 2.9 when constructing certain buildings.
+  * Fixed error when a map asset is reloaded in rare cases.
 
 ## 2.9
 * For players:

src/SMAPI/Metadata/CoreAssetPropagator.cs

@@ -92,7 +92,7 @@ namespace StardewModdingAPI.Metadata
                 bool anyChanged = false;
                 foreach (GameLocation location in this.GetLocations())
                 {
-                    if (this.GetNormalisedPath(location.mapPath.Value) == key)
+                    if (!string.IsNullOrWhiteSpace(location.mapPath.Value) && this.GetNormalisedPath(location.mapPath.Value) == key)
                     {
                         this.Reflection.GetMethod(location, "reloadMap").Invoke();
                         this.Reflection.GetMethod(location, "updateWarps").Invoke();