Maya python subprocess error "subprocess.py line 555: argument of type 'NoneType' is not iterable"

Question

I'm currently trying to get a script to work which should batch export some assets and also render a preview for these. The DCC is maya and the renderengine is redshift. Language is python The script works fine as long as I export one Asset at a time but I cannot pass on multiple asset since the render one rendering process will still run while the next one is trying to be started thus ending the script. I also tried using a subprocess which doesn't work for some reason.

Code:

import subprocess
import RSL_Script_Package.PreviewRender as PreviewRender

subprocess.run([PreviewRender.renderPreview(r"M:\Test","test")],capture_output=True)

print(process)

Output:

! Initiate preview render...

['pCube4']
# Error: TypeError: file G:\Autodesk\Maya2022\Python37\lib\subprocess.py line 555: argument of type 'NoneType' is not iterable # 
// Info:  [Redshift] Redshift for Maya 2022 // 
// Info:  [Redshift] Version 3.0.56, Sep 11 2021 // 
// Info:  [Redshift] Rendering frame 1 (1/1) // 
// Info:  [Redshift] Scene translation time: 0.02s // 
// Info:  [Redshift] ================================================================================================= // 
// Info:  [Redshift] Rendering frame 1... // 
// Info:  [Redshift] AMM enabled // 
// Info:  [Redshift] ================================================================================================= // 
// Info:  [Redshift] License acquired // 
// Info:  [Redshift] License for redshift-core 2021.09 (permanent) // 
// Info:  [Redshift]  // 
// Info:  [Redshift] Device 0 (NVIDIA GeForce RTX 3090) uses OptiX for ray tracing // 
// Info:  [Redshift]  // 
// Info:  [Redshift] Rendering time: 6.1s (1 GPU(s) used) // 
// Info:  [Redshift] Saved file 'M:\Test\test_Preview.png' in 0.22s // 
// Info:  [Redshift] Frame done - total time for frame 1 (1/1): 6.98s // 
// Info:  [Redshift] Rendering done - total time for 1 frames: 7.02s // 
// Info:  [Redshift] License returned  // 

When using another function like "dir" it would give me a proper result as seen here:

Code:

import subprocess
import RSL_Script_Package.PreviewRender as PreviewRender

subprocess.run(["dir"], shell=True ,capture_output=True)

print(process)

Output:

CompletedProcess(args=['dir'], returncode=0)

I guess it has something to do with this error:

# Error: TypeError: file G:\Autodesk\Maya2022\Python37\lib\subprocess.py line 555: argument of type 'NoneType' is not iterable # 

But I have no idea why it happens.

Please help :D