打开动态表单界面报错、

public override void BarItemClick(BarItemClickEventArgs e)
{
base.BarItemClick(e);
if (e.BarItemKey.ToUpperInvariant().Equals("TBCHANGEPRICE"))
{
FormResult formresult = null;
DynamicFormShowParameter param = new DynamicFormShowParameter(); //动态表单唯一标识，由字符串组成
param.FormId = "DM_SalePriceFilter";
param.OpenStyle.ShowType = ShowType.Default;
param.Caption = "改价筛选";
param.ParentPageId = this.View.PageId;
param.PageId = string.Format("{0}_{1}_F7", this.View.PageId, param.FormId);
param.Width = 800;
param.Height = 500;
this.View.ShowForm(param, new Action((result) =>
{
formresult = result;
}));
}

}
报错信息如图：