Fix and harden prospect creation against Blazor runtime crashes

- Handle empty numeric input safely with nullable integer binding.

- Add semaphore locks to prevent double-click invocation errors on mobile.

- Implement global exception handling and user notifications for prospect operations.

FoodsharingSiegen.Server/Dialogs/EditProspectDialog.razor

@@ -11,7 +11,7 @@
     <div class="col">
         <Field>
             <FieldLabel>Foodsharing-ID</FieldLabel>
-            <NumericEdit TValue="int" @bind-Value="Prospect.FsId"></NumericEdit>
+            <NumericEdit TValue="int?" Value="Prospect.FsId" ValueChanged="@((int? v) => Prospect.FsId = v ?? 0)"></NumericEdit>
         </Field>
     </div>
 </div>

FoodsharingSiegen.Server/Dialogs/EditProspectDialog.razor.cs

@@ -82,24 +82,53 @@ namespace FoodsharingSiegen.Server.Dialogs
 
         #region Private Method SaveClick
 
+        private bool _isSaving;
+
         /// <summary>
         ///     Saves the click (a. beging, 31.05.2022)
         /// </summary>
         private async Task SaveClick()
+        {
+            if (_isSaving) return;
+            _isSaving = true;
+            try
             {
                 if (IsUpdateMode)
                 {
                     var updateR = await ProspectService.UpdateAsync(Prospect);
-                if (updateR.Success && OnSuccess != null) await OnSuccess.Invoke();
+                    if (updateR.Success)
+                    {
+                        if (OnSuccess != null) await OnSuccess.Invoke();
+                        await ModalService.Hide();
+                    }
+                    else
+                    {
+                        await Notification.Error(updateR.Exception?.Message ?? "Unbekannter Fehler beim Speichern.", "Fehler");
+                    }
                 }
                 else
                 {
                     var addR = await ProspectService.AddProspectAsync(Prospect);
-                if (addR.Success && OnSuccess != null) await OnSuccess.Invoke();
+                    if (addR.Success)
-            }
+                    {
-
+                        if (OnSuccess != null) await OnSuccess.Invoke();
                         await ModalService.Hide();
                     }
+                    else
+                    {
+                        await Notification.Error(addR.Exception?.Message ?? "Unbekannter Fehler beim Hinzufügen.", "Fehler");
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                await Notification.Error(ex.Message, "Systemfehler");
+            }
+            finally
+            {
+                _isSaving = false;
+            }
+        }
 
         #endregion
     }

FoodsharingSiegen.Server/Pages/Prospects.razor.cs

@@ -56,14 +56,25 @@ namespace FoodsharingSiegen.Server.Pages
 
         #region Private Method CreateProspectAsync
 
+        private bool _isOpeningModal;
+
         /// <summary>
         ///     Asynchronously creates a new prospect by displaying the AddProspectModal dialog and refreshing the prospect list.
         /// </summary>
         /// <returns>A task that represents the asynchronous operation.</returns>
         private async Task CreateProspectAsync()
+        {
+            if (_isOpeningModal) return;
+            _isOpeningModal = true;
+            try
             {
                 await EditProspectDialog.ShowAsync(ModalService, LoadProspects);
             }
+            finally
+            {
+                _isOpeningModal = false;
+            }
+        }
 
         #endregion
 
@@ -88,6 +99,8 @@ namespace FoodsharingSiegen.Server.Pages
         ///     Loads the prospects (a. beging, 11.04.2022)
         /// </summary>
         private async Task LoadProspects()
+        {
+            try
             {
                 var parameter = new GetProspectsParameter
                 {
@@ -99,6 +112,11 @@ namespace FoodsharingSiegen.Server.Pages
 
                 await InvokeAsync(StateHasChanged);
             }
+            catch (Exception ex)
+            {
+                await Notification.Error(ex.Message, "Fehler beim Laden");
+            }
+        }
 
         #endregion
     }