Exception when saving an imagefile

[bitten]

Hello.
I have a problem-saving an image file using c# in Windows Forms App. Environment MS visual and windows 11.

Clip of code where exception appears.(I clipped what I believe is the involved part).

 private void saveImage2()
        {
            if (System.IO.File.Exists(@"D:\bilder\temp.png"))
                  System.IO.File.Delete(@"D:\bilder\temp.png");
           MG.bmp.Save(@"D:\bilder\temp.png", ImageFormat.Png); // exception appears at this line.
            // Dispose of the image files.
          //  MG.bmp.Dispose();
        }

The MG.bmp is a global variable Bitmap bmp which is located in the internal class.(MG).
I use it every time I update image content and others.

Here is the exception text:
System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=ark
StackTrace:
at ark.Form1.saveImage2() in C:\Users\xxxxx\source\repos\bok\ark\Form1.cs:line 1150

Can someone kindly help me get the exception devil off my back after 5 hours of knocking my head?
Thank You.