properly handle switching to a reading type that doesn't have any readings at all

Changelog.md

@@ -13,6 +13,7 @@ This is the changelog for the air quality web interface and its associated HTTP
 ## v0.14
  - Remove the heatmap background overlay :-(
  - Colour device markers based on the latest reading
+ - Properly handle switching to a reading type that doesn't have any readings at all
  - [API] Optimise `list-reading-types` action, which necessitated a database schema update and the removal of the `count` property on returned objects
  - [API] Drastically optimise `device-info` by removing a redundant JOIN
 

client_src/js/LayerDeviceMarkers.mjs

@@ -57,7 +57,14 @@ class LayerDeviceMarkers extends Emitter {
 		
 		// 3: Fetch the latest readings data
 		// --------------------------------------------------------------------
-		let device_values = await this.map_manager.readings_data.fetch(reading_type, datetime);
+		let device_values;
+		try {
+			device_values = await this.map_manager.readings_data.fetch(reading_type, datetime);
+		}
+		catch(error) {
+			alert(error);
+			device_values = new Map();
+		}
 		
 		// 4: Add a marker for each device
 		// --------------------------------------------------------------------