输入一个字符后WPF文本框失去焦点

我对WPF非常陌生。我被指派去修复一个bug。你知道为什么在搜索栏中输入一个字符后，文本框会失去焦点吗?我不知道该发哪个代码来帮助你们。

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition
                Width="100" />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <Label
            Content="Model Name:" />
        <TextBox
            Grid.Column="1"
            Text="{Binding Path=ModelName, UpdateSourceTrigger=PropertyChanged, 
                   ValidatesOnDataErrors=True}"
            IsReadOnly="{Binding IsLoading}"
            Style="{StaticResource FieldTextBox}"
            MaxLength="{Binding ModelNameMaxLength}">
            <TextBox.InputBindings>
                <KeyBinding
                    Key="Enter"
                    Command="vmdls:ModelManagerViewModel.EditModel" />
                <KeyBinding
                    Key="Up"
                    Command="vmdls:ModelManagerViewModel.MoveSelectionUp"/>
                <KeyBinding
                    Key="Down"
                    Command="vmdls:ModelManagerViewModel.MoveSelectionDown" />
            </TextBox.InputBindings>
        </TextBox>
    </Grid>

    private string _modelName;
    public string ModelName
    {
        get { return _modelName; }
        set
        {
            if (_modelName == value)
                return;
            _modelName = value;
            RefreshModels();
        }
    }
     private void RefreshModels()
    {
        if (ModelName.SafeTrim() == string.Empty)
            Models = new ObservableCollection<string>();
        else
        {
            IsLoading = true;
            Models = new ObservableCollection<string>(new string[] { "Loading ..." });
            var workerThread = new Thread(new ThreadStart(GetModelsInternal)) 
          { IsBackground = true };
            workerThread.Start();
        }
    }
    private void GetModelsInternal()
    {
        IWaitIndicator waitIndicator = null;
        var dispatcher = View.ViewElement.Dispatcher;
        dispatcher.Invoke(new ThreadStart(() =>
        {
            waitIndicator = View.GetService<IWaitIndicatorFactory>().CreateWaitIndicator(); 
        }));
        var newModels = new string[] { };
        try
        {
            using (var proxy = new ModelSvcClient())
                newModels = proxy.FindModelsByName(ModelName);
        }
        catch (Exception vx)
        {
            View.GetService<IDisplayMessage>().ShowErrorMessage(vx.Message);
        }
        dispatcher.Invoke(new ThreadStart(() =>
        {
            _models = new ObservableCollection<string>(newModels);
            SelectedModelsItem = _models.FirstOrDefault();
            waitIndicator.Dispose();
            OnPropertyChanged(() => Models);
            IsLoading = false;
        }));
    }