Hello guys , I try to create a Download Manager but when I was running my project and I click in start download button after that this error happen. ______________________________________________________________________________________________________

See the end of this message for details on invoking
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
at DotnetDownloader.frmnewdownload.btnstartdownload_Click(Object sender, EventArgs e) in E:\VisualProject(CSharp)\DownloadMenager\DotnetDownloader\frmnewdownload.cs:line 72
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

************** Loaded Assemblies **************
mscorlib
Assembly Version: 4.0.0.0
Win32 Version: 4.7.3056.0 built by: NET472REL1
CodeBase: file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/mscorlib.dll

DotnetDownloader
Assembly Version: 1.0.0.0
Win32 Version: 1.0.0.0
CodeBase: file:///E:/VisualProject(CSharp)/DownloadMenager/DotnetDownloader/bin/Debug/DotnetDownloader.exe

System.Windows.Forms
Assembly Version: 4.0.0.0
Win32 Version: 4.7.3056.0 built by: NET472REL1
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System.Windows.Forms/v4.0_4.0.0.0__b77a5c561934e089/System.Windows.Forms.dll

System
Assembly Version: 4.0.0.0
Win32 Version: 4.7.3056.0 built by: NET472REL1
CodeBase: file:///C:/Windows/Microsoft.Net/assembly/GAC_MSIL/System/v4.0_4.0.0.0__b77a5c561934e089/System.dll

System.Drawing
Assembly Version: 4.0.0.0
Win32 Version: 4.7.3056.0 built by: NET472REL1
CodeBase: file:///C:/Windows/Mic