How do I fix 'Object reference not set to an instance of an object.'?

public void SetPanelBackgroundImage(Image backgroundImage)
{
      Form1.back_panel.BackgroundImage = backgroundImage;
}

Form1 Form1;

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=ProtoSploit
  StackTrace:
   at ProtoSploit.res.UCs.UC_tab_theme.SetPanelBackgroundImage(Image backgroundImage)