How to load the symbols in FilterRow in WPF DataGrid (SfDataGrid)?

<syncfusion:SfDataGrid x:Name="grid"
                               AllowEditing="True"
                               FilterRowPosition="Top"
                              AutoGenerateColumns="False"                              
                              ColumnSizer="Star"
                              ItemsSource="{Binding Source}
 
   <syncfusion:SfDataGrid.Columns>
    
    <syncfusion:GridTextColumn HeaderText="Income" TextAlignment="Right" 
                            FilterRowEditorType="CustomNumericFilterRowRenderer"
                            MappingName="Salary" 
                            DisplayBinding="{Binding Salary,StringFormat={}{0:c}}"/>
 
  </syncfusion:SfDataGrid.Columns>

public class GridFilterRowCellExt : GridFilterRowCell
{
    private const string EqualsSymbol = "=";
    private const string NotEquals = "<>";
    private const string After = ">";
    private const string AfterOrEqual = ">=";
    private const string Before = "<";
    private const string BeforeOrEqual = "<=";
 
 
    public override void OpenFilterOptionPopup()
    {
 
        base.OpenFilterOptionPopup();
 
        if (DataColumn.GridColumn is GridTextColumn && DataColumn.GridColumn.MappingName == "Salary")
        {
            FilterOptionsList.ItemsSource = IncomeOptions();
        }
    }
 
 
    private ObservableCollection<string> IncomeOptions()
    {
        var list = new ObservableCollection<string>
        {
            EqualsSymbol,
            NotEquals,
            After,
            AfterOrEqual,
            Before,
            BeforeOrEqual
        };
        return list;
    }
}
 

public class CustomNumericFilterRowRenderer : GridFilterRowCellRenderer<ContentControl, TextBox>
{
 
    private TextBox textBox;
    public override void OnInitializeDisplayElement(DataColumnBase dataColumn, ContentControl uiElement, object dataContext)
    {
        GridColumn gridColumn = dataColumn.GridColumn;
        Binding binding1 = new Binding
        {
            Path = new PropertyPath("FilterRowText"),
            Mode = BindingMode.TwoWay,
            Source = gridColumn,
            UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged
        };
 
    }
 
    protected override void OnWireEditUIElement(TextBox uiElement)
    {
        uiElement.TextChanged += UiElement_TextChanged; ;
    }
 
    private void UiElement_TextChanged(object sender, TextChangedEventArgs e)
    {
        var textBox = sender as TextBox;
        ProcessSingleFilter(textBox.Text);
 
    }
 
    protected override void OnUnwireEditUIElement(TextBox uiElement)
    {
        uiElement.TextChanged -= UiElement_TextChanged;
    }
 
    protected override void OnEditElementLoaded(object sender, RoutedEventArgs e)
    {
        textBox = sender as TextBox;
        var uiElement = textBox;
        uiElement.Focus();
        uiElement.TextChanged += UiElement_TextChanged;
    }
 
      
 
    public override void OnFilterRowConditionChanged(string filterRowCondition)
    {
        filterRowCondition = GetFilterRowCondition(filterRowCondition);
 
        base.OnFilterRowConditionChanged(filterRowCondition);
        ProcessSingleFilter(textBox.Text);
    }
 
    private string GetFilterRowCondition(string filterType)
    {
        if (filterType == "=")
            return GridResourceWrapper.Equalss;
        else if (filterType == "<>")
            return GridResourceWrapper.NotEquals;
        else if (filterType == ">")
            return GridResourceWrapper.GreaterThan;
        else if (filterType == ">=")
            return GridResourceWrapper.GreaterThanorEqual;
        else if (filterType == "<")
            return GridResourceWrapper.LessThan;
        else if (filterType == "<=")
            return GridResourceWrapper.LessThanorEqual;
        return GridResourceWrapper.Equalss;
    }
 
    public override string GetFilterText(List<FilterPredicate> filterPredicates)
    {
        return textBox.Text;
    }
 
 
    private new List<FilterPredicate> GetFilterPredicates(object filterValue)
    {
        if (filterValue == null)
            filterValue = textBox.Text ?? "";
        
 
        FilterType filterType;
        object value = filterValue;
        switch (FilterRowCell.DataColumn.GridColumn.FilterRowCondition)
        {
            case FilterRowCondition.Equals:
                filterType = FilterType.Equals;
                break;
            case FilterRowCondition.NotEquals:
                filterType = FilterType.NotEquals;
                break;
            case FilterRowCondition.Before:
                filterType = FilterType.LessThan;
                break;
            case FilterRowCondition.BeforeOrEqual:
                filterType = FilterType.LessThanOrEqual;
                break;
            case FilterRowCondition.After:
                filterType = FilterType.GreaterThan;
                break;
            case FilterRowCondition.AfterOrEqual:
                filterType = FilterType.GreaterThanOrEqual;
                break;
            case FilterRowCondition.Null:
                filterType = FilterType.Equals;
                value = null;
                break;
            case FilterRowCondition.NotNull:
                filterType = FilterType.NotEquals;
                value = null;
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }
 
        return new List<FilterPredicate>{new FilterPredicate
        {
            FilterBehavior = FilterBehavior.StronglyTyped,
            FilterMode = ColumnFilter.Value,
            FilterType = filterType,
            FilterValue = value,
            IsCaseSensitive = false,
            PredicateType = PredicateType.OrElse
        }};
    }
}
 

this.dataGrid.FilterRowCellRenderers.Add("CustomNumericFilterRowRenderer", new CustomNumericFilterRowRenderer());
this.dataGrid.RowGenerator = new CustomRowGenerator(dataGrid);