Copy Selected Text from a combobox to clipboard

i got a little problem with my program...

I have a combobox with 4 variables and a button to copy the text in the combobox.

If i select the first one in the combobox and press the copy button the test ends and vs saýs:

System.ArgumentNullException ist aufgetreten.
HResult=0x80004003
Nachricht = Der Wert darf nicht NULL sein.
Parametername: text
Quelle = System.Windows.Forms
Stapelüberwachung:
bei System.Windows.Forms.Clipboard.SetText(String text, TextDataFormat format)
bei System.Windows.Forms.Clipboard.SetText(String text)
bei WindowsFormsApp1.Form1.button2_Click(Object sender, EventArgs e) in 
C:\Users\user\source\repos\WindowsFormsApp1\WindowsFormsApp1\Form1.cs: Zeile34
bei System.Windows.Forms.Control.OnClick(EventArgs e)
bei System.Windows.Forms.Button.OnClick(EventArgs e)
bei System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
bei System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
bei System.Windows.Forms.Control.WndProc(Message& m)
bei System.Windows.Forms.ButtonBase.WndProc(Message& m)
bei System.Windows.Forms.Button.WndProc(Message& m)
bei System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
bei System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
bei System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
bei System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
bei System.Windows.Forms.Application.ComponentManager. 
System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.
FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
bei System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 
reason, ApplicationContext context)
bei System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 
reason, ApplicationContext context)
bei System.Windows.Forms.Application.Run(Form mainForm)
bei WindowsFormsApp1.Program.Main() in C:\Users\user\source\repos\WindowsFormsApp1\WindowsFormsApp1\Program.cs: Zeile19

With just the exception text it is a little difficult to tell, but it sounds like you may be trying to copy a null or empty string to the clipboard. C# does not allow you to copy an empty string to the clipboard, and will throw this error.