Error in copying of an class object in another window of application

Error in copying of an class object in another window of application

I am stuck. Even don't know what it might be. I trying to use properties
of an object in Main Window on its copy in child window, but this action
gives me an error. Link to download My project
Fragment of code where an error appears: public double this[int index_row,
int index_column] { get { return
double.Parse(data.Rows[index_row][index_column] + ""); } set {
data.Rows[index_row].SetField<double>(index_column, value); } }