New: Warn if UI won't update due to SignalR errors

frontend/src/Store/Selectors/createHealthCheckSelector.js

@@ -0,0 +1,24 @@
+import { createSelector } from 'reselect';
+
+function createHealthCheckSelector() {
+  return createSelector(
+    (state) => state.system.health,
+    (state) => state.app,
+    (health, app) => {
+      const items = [...health.items];
+
+      if (!app.isConnected) {
+        items.push({
+          source: 'UI',
+          type: 'warning',
+          message: 'Could not connect to SignalR, UI won\'t update',
+          wikiUrl: 'https://github.com/Radarr/Radarr/wiki/Health-Checks#could-not-connect-to-signalr'
+        });
+      }
+
+      return items;
+    }
+  );
+}
+
+export default createHealthCheckSelector;