Saving drawning with saveFileDialog issue. C# GUI

Question

I have to do "Paint-similar" application, and I have made brush,pen, etc. , so now I want to save my drawing and I have issue with saveFileDialog. It catches:

NullReferenceException was caught. Object reference not set to an instance of an object. Troubleshoot tips: Check to determine if the object is null before calling the method. Use the "new" keyword to create an object instance.

Here is what I've done for openFileDialog:

private void openToolStripMenuItem_Click(object sender, EventArgs e)        
{
    openFileDialog1.Title = "Open an image";
    openFileDialog1.Filter = "JPEG|*.jpeg|GIF|*.gif|BMP|*.bmp|TIFF|*.tiff|PNG|*.png|JPG|*.jpg";
    if (openFileDialog1.ShowDialog() == DialogResult.OK)
    {
        try
        {
            file = Image.FromFile(openFileDialog1.FileName);
            pictureBox1.Image = file;
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    } 
}

Here is what I've done for saveFileDialog:

private void saveToolStripMenuItem_Click(object sender, EventArgs e)
{

    saveFileDialog1.Title = "Saving image";
    saveFileDialog1.Filter = "JPEG|*.jpeg";
    if(saveFileDialog1.ShowDialog()==DialogResult.OK)
    {
        try
        {
           file.Save(saveFileDialog1.FileName);
        }
        catch (Exception ex)
        {
           MessageBox.Show(ex.Message);
        }
    }
}