Fix uncaught TypeError in "Show on map" operation.

src/core/operations/ShowOnMap.mjs

@@ -71,6 +71,16 @@ class ShowOnMap extends Operation {
             }
             latLong = latLong.replace(/[,]$/, "");
             latLong = latLong.replace(/°/g, "");
+
+            // The map requires a latitude and longitude pair. If the conversion only produced a
+            // single value (e.g. because the chosen input delimiter didn't match the input), bail
+            // out with a helpful message rather than passing it on to the map, which would throw an
+            // uncaught TypeError in the browser.
+            const coords = latLong.split(",").map(v => v.trim());
+            if (coords.length !== 2 || coords.some(v => v === "" || isNaN(Number(v)))) {
+                throw new OperationError(`Could not show coordinates '${latLong}' on the map. Expected a latitude and longitude pair - check that the input format and delimiter are correct.`);
+            }
+
             return latLong;
         }
         return input;

tests/operations/tests/ShowOnMap.mjs

@@ -0,0 +1,39 @@
+/**
+ * Show on map tests
+ *
+ * @author Leon Zandman [leon@wirwar.com]
+ *
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Show on map: valid coordinate pair",
+        input: "51.5007, -0.1246",
+        // The presented output is the Leaflet map HTML; just check the coordinates made it through.
+        expectedMatch: /51\.5007,-0\.1246/,
+        recipeConfig: [
+            {
+                op: "Show on map",
+                args: [13, "Auto", "Auto"]
+            },
+        ],
+    },
+    {
+        // Regression test: a comma-separated input with the delimiter set to "\n" used to be
+        // mis-detected as a single Degrees Decimal Minutes value (1° 24' = 1.4°), producing a single
+        // coordinate. That single value was then passed to Leaflet's setView([1.4], ...), throwing
+        // an uncaught "Cannot read properties of null (reading 'lat')" TypeError in the browser.
+        name: "Show on map: single value is rejected with a helpful error",
+        input: "1, 24",
+        expectedOutput: "Could not show coordinates '1.4' on the map. Expected a latitude and longitude pair - check that the input format and delimiter are correct.",
+        recipeConfig: [
+            {
+                op: "Show on map",
+                args: [13, "Auto", "\\n"]
+            },
+        ],
+    },
+]);