System.invalidoperationexception error and I cant seem to fix it

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C#

System.InvalidOperationException
HResult=0x80131509
Message=Fill: SelectCommand.Connection property has not been initialized.
Source=<cannot evaluate="" the="" exception="" source="">
StackTrace:
<cannot evaluate="" the="" exception="" stack="" trace="">




using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Web;

namespace project
{
public class DAL
{
private SqlConnection connection;
private SqlCommand command;
private SqlDataAdapter adapter;
private string SQLstr;
private string connectionString;

public DAL()
{
connectionString = string.Format(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\zafal\source\repos\finalprojectrecipe\finalprojectrecipe\db.mdf;Integrated Security=True;Connect Timeout=30");
SQLstr = "Select *from Users";
command = new SqlCommand(SQLstr, connection);
adapter = new SqlDataAdapter(command);
}

public DataSet GetDataSetWithSql(string strSQL)
{
DataSet dataset = new DataSet();
command.CommandText = strSQL;
adapter.SelectCommand = command;
adapter.SelectCommand = command;

adapter.Fill(dataset);
return dataset;
}

public int insert(string fname, string lname, string email, string password)
{
int success = -1;

using(SqlConnection connection = new SqlConnection(connectionString))
{
connection.Open();
string sql = "INSERT INTO Users(fname,lname,EMAIL,password,is admin) VALUES(@pass,@email,@param3,@param4,@param5,@param6)";
using (SqlCommand cmd = new SqlCommand(sql, connection))
{
cmd.Parameters.Add("@email", SqlDbType.NVarChar, 40).Value = email;
cmd.Parameters.Add("@pass", SqlDbType.NVarChar, 40).Value = password;
cmd.Parameters.Add("@param3", SqlDbType.NVarChar, 20).Value = fname;
cmd.Parameters.Add("@param4", SqlDbType.NVarChar, 20).Value = lname;
cmd.Parameters.Add("@param5", SqlDbType.Int).Value = 0;
cmd.Parameters.Add("@param6", SqlDbType.TinyInt).Value = 0;
cmd.CommandType = CommandType.Text;
success = cmd.ExecuteNonQuery();

What I have tried:

i have tried chnaging the code but im a newbie and im not sure what to do