Object reference not set to an instance of an object. Why? If the values ​​are not empty?

Question

WPF

I send data to the server. They are not empty. I receive a response from the server and after 1-2 seconds I get the exception Object reference not set to an instance of an object.
What am I doing wrong? And from what it arises, if the data is not empty, as far as I understand?

Answer 1

If you replace it with this code, it works

string strValue = lstbox.SelectedValue as string;
string MyValue= listBoxfilesAndDirectoriesInServerList.SelectedValue.ToString() ;
MessageBox.Show(MyValue);

Answer 2

Add a check

if(listBoxfilesAndDirectoriesInServerList.SelectedValue != null)
{
   //....
}