dashboard: surface lazy-load errors via BrowseLoadState.Error

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/Pages/BrowsePage.razor

@@ -232,6 +232,11 @@
     private async Task LoadChildrenAsync(DashboardBrowseNode node)
     {
         BrowseLevelResult result = BrowseService.GetChildren(node.Object.GobjectId, new BrowseFilterArgs());
+        if (!string.IsNullOrEmpty(result.Error))
+        {
+            throw new InvalidOperationException(result.Error);
+        }
+
         node.Children.Clear();
         foreach (DashboardBrowseNode child in result.Nodes)
         {

src/ZB.MOM.WW.MxGateway.Tests/Dashboard/DashboardBrowseServiceTests.cs

@@ -82,6 +82,21 @@ public sealed class DashboardBrowseServiceTests
         Assert.Equal(12UL, after.CacheSequence);
     }
 
+    /// <summary>Verifies that <see cref="DashboardBrowseService.GetChildren"/> returns an
+    /// empty node list with a non-empty <c>Error</c> when the cache has not yet loaded
+    /// (i.e. <see cref="GalaxyHierarchyCacheEntry.HasData"/> is false).</summary>
+    [Fact]
+    public void GetChildren_CacheNotLoaded_ReturnsErrorResult()
+    {
+        StubGalaxyHierarchyCache cache = new(GalaxyHierarchyCacheEntry.Empty);
+        DashboardBrowseService service = new(cache);
+
+        BrowseLevelResult result = service.GetChildren(parentGobjectId: 1, new BrowseFilterArgs());
+
+        Assert.Empty(result.Nodes);
+        Assert.False(string.IsNullOrEmpty(result.Error));
+    }
+
     private static IReadOnlyList<GalaxyObject> CreateObjects()
     {
         // Fixture: an Area "Plant" (id 1, parent 0, IsArea=true) containing one