How to retrieve the ImagePlus Object from ImageJ ImageCalculator

Question

i dont understand how can i use the run method of the ij.plugin ImageCalculator Class to return a ImagePlus object instead of void. I tested the script from here: https://imagej.nih.gov/ij/developer/api/ij/ij/plugin/ImageCalculator.html in my python script and got: AttributeError: 'NoneType' object has no attribute 'show'.

My script:

import os
from ij import WindowManager, IJ
from ij.plugin import ImageCalculator as ic

dir = "path to my files"

imp1 = WindowManager.getImage(dir + "bgn0.tif")
for file in os.listdir(dir):
    filename = file
    if filename.endswith(".tif"):
        imp2 = WindowManager.getImage(filename)
        imp3 = ic().run("subtract create", imp1, imp2);
        imp3.show();
        IJ.saveAsTiff(imp3, dir + "\subtracted\\" + filename)

I aim for using this script with ImageJ to use background subtraction on all files in a target folder. Finally i want to save the images in the "subtracted" sub folder.