fix: release the alarm drain latch on escape; cancel before disposing the page poll timers

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Fleet.razor

@@ -114,6 +114,10 @@ else
     // goes away aborts its query instead of running it to completion against a component that's gone.
     private readonly CancellationTokenSource _cts = new();
 
+    // Set first thing in DisposeAsync. A tick that resumes after teardown must not call
+    // StateHasChanged — the renderer is gone and it throws, inside a task nobody awaits.
+    private volatile bool _disposed;
+
     protected override async Task OnInitializedAsync()
     {
         await LoadAsync();
@@ -170,7 +174,10 @@ else
             // Release first: a StateHasChanged on a torn-down circuit must never leave the guard set.
             Volatile.Write(ref _tickRunning, 0);
             _refreshing = false;
-            StateHasChanged();
+            // This one sits OUTSIDE the try, so it has no catch to fall into — and a render
+            // request against a disposed renderer throws. Skip it once the page is gone; there
+            // is nothing left to render to.
+            if (!_disposed) StateHasChanged();
         }
     }
 
@@ -186,13 +193,20 @@ else
 
     public async ValueTask DisposeAsync()
     {
+        _disposed = true;
         // Cancel before draining so a tick already in flight aborts its queries rather than running
         // them to completion against a component that's gone.
         _cts.Cancel();
         // Drain the timer so an in-flight callback can't touch a component that's already gone
         // (System.Threading.Timer's async dispose awaits any in-flight callback — .NET 6+).
         if (_timer is not null) await _timer.DisposeAsync();
-        _cts.Dispose();
+        // ...but the callback body is `_ = InvokeAsync(LoadAsync)`, so that await covered only the
+        // SCHEDULING of the tick, never the tick itself: a query can still be running against
+        // _cts.Token right now. Disposing the source out from under it turns the clean
+        // cancellation the line above arranged into an ObjectDisposedException mid-query, so skip
+        // the dispose while a tick holds it. An undisposed CancellationTokenSource with no timer
+        // and no materialised WaitHandle costs nothing and is collected with the component.
+        if (Volatile.Read(ref _tickRunning) == 0) _cts.Dispose();
     }
 
     private sealed record NodeRow(