private void FilterFormLoad(object sender, EventArgs e)
 {
     OnDropDownList.DataSource = Enum.GetValues(typeof(FilterKeyEnum));
     OperatorDropDownList.DataSource = Enum.GetValues(typeof(FilterOperatorEnum));
     OnDropDownList.Enabled = false;
     OperatorDropDownList.Enabled = false;
     ValueTextBox.Enabled = false;
     RemoveButton.Enabled = false;
     ApplyButton.Enabled = false;
     FiltersTreeView.Nodes.Clear();
     FiltersTreeView.Nodes.AddRange(TreeNodeMapper.MapFilters(_filters));
 }