diff --git a/ChatController/ChatController.csproj b/ChatController/ChatController.csproj
index f9cd21c..406f321 100644
--- a/ChatController/ChatController.csproj
+++ b/ChatController/ChatController.csproj
@@ -46,6 +46,7 @@
       <HintPath>..\PrototypChat\packages\RestSharp.105.2.3\lib\net45\RestSharp.dll</HintPath>
     </Reference>
     <Reference Include="System" />
+    <Reference Include="System.Configuration" />
     <Reference Include="System.Core" />
     <Reference Include="System.Drawing" />
     <Reference Include="System.Web" />
diff --git a/ChatController/ChatMainControl.xaml.cs b/ChatController/ChatMainControl.xaml.cs
index b39cb63..6be474c 100644
--- a/ChatController/ChatMainControl.xaml.cs
+++ b/ChatController/ChatMainControl.xaml.cs
@@ -319,7 +319,7 @@ namespace ChatController
                 _ContainingWindow.Activate();
 
                 var selectedContact = _ContactList?.FirstOrDefault(f => f?.Contact.GroupId == pGroupId);
-                if(!(selectedContact is null))
+                if(!(selectedContact?.Contact is null))
                 {
                     SelectContact(selectedContact.Contact);
                 }
@@ -374,83 +374,93 @@ namespace ChatController
                 return;
             }
 
-            if(Clientlist?.SelectedItem is ContactDependencyObject selectedContactDependencyObject)
+            if(Clientlist?.SelectedItem is ContactDependencyObject selectedContactDependencyObject && !(selectedContactDependencyObject.Contact is null))
             {
                 SelectContact(selectedContactDependencyObject.Contact);
             }
         }
 
+        // ToDo: Hier kommt es immer wieder zu einer NullPointerException
         private void SelectContact(Contact contact)
         {
-            if(contact is null)
+            try
             {
-                return;
-            }
-
-            ChatMessageFileWrapper = null;
-
-            ShouldInterruptContactsThread = true;
-
-            StartWaitingImmediately();
-
-            _ScrollPrueferAktivieren = false;
-
-            if(contact.HasUnreadMessages)
-            {
-                contact.HasUnreadMessages = false;
-
-                Clientlist.Items.Refresh();
-                OnPropertyChanged(nameof(ContactList));
-            }
-
-            var currentContact = contact;
-            contact.IsNewMessage = false;
-
-            var shouldChangeIcon = _ContactList.Any(contactDependencyObject => contactDependencyObject.Contact.IsNewMessage);
-            if(shouldChangeIcon)
-            {
-                ContainingWindow.Icon = Utils.ConvertIconToImageSource(Resource.ownchat_favicon);
-            }
-
-            CurrentContact = currentContact;
-
-            OnPropertyChanged(nameof(IsSendButtonEnabled));
-
-            foreach(ContactDependencyObject item in Clientlist.Items)
-            {
-                if(item.Contact.Equals(contact))
+                if(contact is null)
                 {
-                    Clientlist.SelectedItem = item;
+                    return;
                 }
-            }
 
-            _ChatMessages.Clear();
-            OnPropertyChanged(nameof(_ChatMessages));
+                ChatMessageFileWrapper = null;
 
-            Chat.LoadChatMessagesForContactAsync(currentContact, GetFirstMessage(), chatMessages =>
-            {
-                this.Dispatch(() =>
+                ShouldInterruptContactsThread = true;
+
+                // Zeigt die Warteanimation (das Rädchen)
+                StartWaitingImmediately();
+
+                _ScrollPrueferAktivieren = false;
+
+                if(contact.HasUnreadMessages)
                 {
-                    _ChatMessages.Clear();
-                    _ChatMessages.AddRangeIfElementsNotIn(chatMessages);
-                    OnPropertyChanged(nameof(_ChatMessages));
+                    contact.HasUnreadMessages = false;
 
-                    WpfUtils.ScrollToBottomOfListBox(ChatListBox);
+                    Clientlist?.Items.Refresh();
+                    OnPropertyChanged(nameof(ContactList));
+                }
 
-                    ChatListBox.ContextMenu = Chat.ErstelleKontextMenue();
-                    SetContextHandler();
-                    _ScrollPrueferAktivieren = true;
+                var currentContact = contact;
 
-                    ListenForMessages();
-                    Cursor = Cursors.Arrow;
+                contact.IsNewMessage = false;
 
-                    WriteToNewSyncFile();
+                var shouldChangeIcon = _ContactList.Any(contactDependencyObject => contactDependencyObject.Contact.IsNewMessage);
+                if(shouldChangeIcon)
+                {
+                    ContainingWindow.Icon = Utils.ConvertIconToImageSource(Resource.ownchat_favicon);
+                }
 
-                    ShouldInterruptContactsThread = false;
+                CurrentContact = currentContact;
 
-                    EndWaiting();
+                OnPropertyChanged(nameof(IsSendButtonEnabled));
+
+                foreach(ContactDependencyObject item in Clientlist.Items)
+                {
+                    if(item.Contact.Equals(contact))
+                    {
+                        Clientlist.SelectedItem = item;
+                    }
+                }
+
+                _ChatMessages.Clear();
+                OnPropertyChanged(nameof(_ChatMessages));
+
+                Chat.LoadChatMessagesForContactAsync(currentContact, GetFirstMessage(), chatMessages =>
+                {
+                    this.Dispatch(() =>
+                    {
+                        _ChatMessages.Clear();
+                        _ChatMessages.AddRangeIfElementsNotIn(chatMessages);
+                        OnPropertyChanged(nameof(_ChatMessages));
+
+                        WpfUtils.ScrollToBottomOfListBox(ChatListBox);
+
+                        ChatListBox.ContextMenu = Chat.ErstelleKontextMenue();
+                        SetContextHandler();
+                        _ScrollPrueferAktivieren = true;
+
+                        ListenForMessages();
+                        Cursor = Cursors.Arrow;
+
+                        WriteToNewSyncFile();
+
+                        ShouldInterruptContactsThread = false;
+
+                        EndWaiting();
+                    });
                 });
-            });
+            }
+            catch(Exception exception)
+            {
+                throw;
+            }
         }
 
         private void SetContextHandler()
diff --git a/ChatController/HauptKlassen/Chat.cs b/ChatController/HauptKlassen/Chat.cs
index d19ad3b..6a62a5f 100644
--- a/ChatController/HauptKlassen/Chat.cs
+++ b/ChatController/HauptKlassen/Chat.cs
@@ -357,20 +357,20 @@ namespace ChatController.HauptKlassen
                 return result;
             }
 
-            var messageId = 0;
-            var userName = ChatDaten.LoggedInUser.Response.User.UserName;
-            var sendTime = DateTime.Now;
-            var fileName = Path.GetFileName(pathToScaledFile);
-            var imageFile = GetImageFile(pathToScaledFile, out var fileSize);
-            var isImage = Constants.ImageFileExtensions.Contains(Path.GetExtension(pathToScaledFile).ToUpperInvariant());
-            var messageText = string.IsNullOrWhiteSpace(chatBoxText) ? fileName : chatBoxText;
-            var filePath = isImage ? null : pathToScaledFile;
-            var originalImage = isImage ? pathToScaledFile : null;
-            var imageName = isImage ? fileName : null;
+            var messageId       = 0;
+            var userName        = ChatDaten.LoggedInUser.Response.User.UserName;
+            var sendTime        = DateTime.Now;
+            var fileName        = Path.GetFileName(pathToScaledFile);
+            var imageFile       = GetImageFile(pathToScaledFile, out var fileSize);
+            var isImage         = Constants.ImageFileExtensions.Contains(Path.GetExtension(pathToScaledFile).ToUpperInvariant());
+            var messageText     = string.IsNullOrWhiteSpace(chatBoxText) ? fileName : chatBoxText;
+            var filePath        = isImage ? null : pathToScaledFile;
+            var originalImage   = isImage ? pathToScaledFile : null;
+            var imageName       = isImage ? fileName : null;
             var linkToThumbnail = isImage ? null : $"{ChatDaten.ServerUrl}/document.png";
-            var logo = isImage ? imageFile : null;
+            var logo            = isImage ? imageFile : null;
             var chatMessageType = isImage ? ChatMessageType.Image : ChatMessageType.Document;
-            var senderId = ChatDaten.LoggedInUser.Response.User.Oid;
+            var senderId        = ChatDaten.LoggedInUser.Response.User.Oid;
 
             var chatMessage = new ChatMessage(groupId, messageId, userName, messageText, sendTime, true, fileSize, filePath, originalImage, imageName, linkToThumbnail, logo, chatMessageType, senderId);
 
@@ -925,11 +925,17 @@ namespace ChatController.HauptKlassen
 
         private static List<ChatMessage> AddSeparators(List<ChatMessage> chatMessages)
         {
+            if(chatMessages is null)
+            {
+                chatMessages = new List<ChatMessage>();
+            }
+
             var messagesWithoutSeparators = chatMessages.Where(message => !message.IsSeparator).OrderBy(message => message.SendTime).ToList();
 
             var separators = new List<ChatMessage>();
 
             var previousMessage = messagesWithoutSeparators.OrderBy(message => message.SendTime).FirstOrDefault();
+
             foreach(var message in messagesWithoutSeparators.OrderBy(message => message.SendTime))
             {
                 if(!message.Equals(previousMessage) && !(previousMessage is null))
diff --git a/ChatController/Properties/AssemblyInfo.cs b/ChatController/Properties/AssemblyInfo.cs
index 96c7324..b8fb540 100644
--- a/ChatController/Properties/AssemblyInfo.cs
+++ b/ChatController/Properties/AssemblyInfo.cs
@@ -1,5 +1,4 @@
 using System.Reflection;
-using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 // Allgemeine Informationen über eine Assembly werden über die folgenden
diff --git a/ChatController/packages.config b/ChatController/packages.config
index e327997..a63cb4d 100644
--- a/ChatController/packages.config
+++ b/ChatController/packages.config
@@ -1,5 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="ClrHeapAllocationAnalyzer" version="3.0.0" targetFramework="net45" />
   <package id="RestSharp" version="105.2.3" targetFramework="net45" />
 </packages>
\ No newline at end of file
diff --git a/PrototypChat/ExceptionWindow.xaml.cs b/PrototypChat/ExceptionWindow.xaml.cs
index 7db6408..082d0d9 100644
--- a/PrototypChat/ExceptionWindow.xaml.cs
+++ b/PrototypChat/ExceptionWindow.xaml.cs
@@ -7,6 +7,7 @@ namespace ownChat
         public ExceptionWindow(Exception exception)
         {
             InitializeComponent();
+
             ExceptionViewControl.Exception = exception;
         }