Fix error when using non-swisstopo maps

src/mixins/mapstyle.js

@@ -152,9 +152,11 @@ export default {
           map.setLayoutProperty('snowcover-partial', 'visibility', 'none')
           map.setLayoutProperty('snowcover', 'visibility', 'none')
         }
+        if (map.getLayer('water_polygon')) {
           map.setLayoutProperty('water_polygon', 'visibility', 'visible')
         }
       }
+    }
   },
   data () {
     return {