TypeError: expected str, bytes or os.PathLike object, not NoneType
🐛 Bug
I am summarizing the source of the issue to speedup the fix.
After this line of code

  
    
      pytorch-lightning/pytorch_lightning/accelerators/ddp_backend.py
    
    
         Line 119
      in
      90929fa
    
  
  
    

        
          
           env_copy['PL_GLOBAL_SEED'] = os.environ.get('PL_GLOBAL_SEED') 
        
    
  


I have that env_copy['PL_GLOBAL_SEED'] is None and having an environment variable set to None breaks subprocess.Popen here

  
    
      pytorch-lightning/pytorch_lightning/accelerators/ddp_backend.py
    
    
         Line 127
      in
      90929fa
    
  
  
    

        
          
           proc = subprocess.Popen(command, env=env_copy, cwd=cwd) 
        
    
  


My fix at the moment is to add
if env_copy['PL_GLOBAL_SEED'] is None:
                del env_copy['PL_GLOBAL_SEED']

after

  
    
      pytorch-lightning/pytorch_lightning/accelerators/ddp_backend.py
    
    
         Line 119
      in
      90929fa
    
  
  
    

        
          
           env_copy['PL_GLOBAL_SEED'] = os.environ.get('PL_GLOBAL_SEED') 
        
    
  


Environment
* CUDA:
	- GPU:
	- available:         False
	- version:           10.2
* Packages:
	- numpy:             1.18.5
	- pyTorch_debug:     False
	- pyTorch_version:   1.6.0
	- pytorch-lightning: 0.10.0rc1
	- tqdm:              4.48.0
* System:
	- OS:                Linux
	- architecture:
		- 64bit
		- 
	- processor:         x86_64
	- python:            3.7.7
	- version:           #100-Ubuntu SMP Wed Apr 22 20:32:56 UTC 2020