Themes(sauna): add sauna question to sports centre

assets/layers/sports_centre/sports_centre.json

@@ -1446,7 +1446,8 @@
         "it": "Quali sport si praticano in questa struttura?",
         "cs": "Jaké sporty se provozují na tomto sportovišti?"
       }
-    }
+    },
+    "sauna_at_leisure.sauna_grouped"
   ],
   "filter": [
     "open_now"

assets/themes/sauna/sauna.json

@@ -37,6 +37,32 @@
         }
       }
     },
+    {
+      "builtin": "sports_centre",
+      "override": {
+        "minzoom": 8,
+        "id": "sports_centre_with_sauna",
+        "presets=": [],
+        "filter=":[],
+        "source": {
+          "osmTags": {
+            "and+": [
+              "sauna=yes"
+            ]
+          }
+        }
+      }
+    },
+    {
+      "builtin": "sports_centre",
+      "override": {
+        "name": null,
+        "minzoom": 18,
+        "filter": {
+          "sameAs": "sports_centre_with_sauna"
+        }
+      }
+    },
     {
       "builtin": "shops",
       "override": {

src/Models/ThemeConfig/Conversion/PrepareTheme.ts

@@ -505,7 +505,11 @@ class WarnForUnsubstitutedLayersInTheme extends DesugaringStep<ThemeConfigJson>
             if (layer["builtin"] !== undefined) {
                 continue
             }
-            if (layer["source"]["geojson"] !== undefined) {
+            if(layer["override"]!==undefined){
+                context.err("Got an `override` block without a `builtin`-specification")
+                continue
+            }
+            if (layer["source"]?.["geojson"] !== undefined) {
                 // We turn a blind eye for import layers
                 continue
             }