Value cannot be null parameter name key? In C#

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C#7.0

public bool getSessions()
{
string S = comboBox1.Text.ToString();
try

{

string query = "SELECT 'ALTER SYSTEM KILL SESSION ' || '''' || sid || ',' || serial# || '''' ||' IMMEDIATE; ' KILL_IT, username, status, schemaname, module, osuser, program, action, logon_time, s.sid," +
" s.SERIAL# " + "FROM v$session s " + "WHERE s.status <> 'KILLED' and upper(s.username) like upper('%" + comboBox1.Text.Trim() + "%') " +
"order by s.username, s.logon_time";

using (OracleConnection con = new OracleConnection(conString))
{
con.Open();
OracleCommand cmd = con.CreateCommand();
cmd.CommandText = query;


using (var reader = cmd.ExecuteReader())
{
DataTable dt = new DataTable();

OracleDataAdapter da = new OracleDataAdapter(cmd);
da.Fill(dt);
DataTable tempDT = new DataTable();
tempDT = dt.DefaultView.ToTable(true, "KILL_IT", "USERNAME", "SCHEMANAME", "OSUSER", "PROGRAM", "LOGON_TIME");
dataGridView1.DataSource = tempDT;

}

con.Close();
return true;
}

}
catch (Exception ex)
{
MessageBox.Show(ex.HResult.ToString());
MessageBox.Show(ex.Message);
return false;
}
}

What I have tried:

have problem

value cannot be null parameter name key?in c#