I have asked [here][1] for a solution how single video images can be extracted very fast from avi files.

[Piotr Wendykier][2] gave a solution for version 11.3.

No I am using Mathematica 12 and repeated his code, whicht does not work any more. 

Why is 

    i = MediaTools`Private`$MFReadNextFrame[]

not reading an image out of the video as in version 11.3?

    In[1]:= file = 
     URLSave["http://mirrors.standaloneinstaller.com/video-sample/\
    Panasonic_HDC_TM_700_P_50i.avi", "c:\\tmp\\sample.avi"]
    
    Out[1]= "c:\\tmp\\sample.avi"
    
    In[2]:= Needs["MediaTools`"]
    
    In[3]:= MediaTools`Private`$MFInitReader[file]
    
    Out[3]= MediaTools`Private`$MFInitReader["c:\\tmp\\sample.avi"]
    
    In[4]:= AbsoluteTiming[i = MediaTools`Private`$MFReadNextFrame[];]
    
    Out[4]= {1.9*10^-6, Null}
    
    In[5]:= MediaTools`Private`$MFFinalizeReader[]
    
    Out[5]= MediaTools`Private`$MFFinalizeReader[]
    
    In[6]:= ImageDimensions[i]
    
    During evaluation of In[6]:= ImageDimensions::imginv: Expecting an image or graphics instead of MediaTools`Private`$MFReadNextFrame[].
    
    Out[6]= ImageDimensions[MediaTools`Private`$MFReadNextFrame[]]


  [1]: https://mathematica.stackexchange.com/questions/186990/fast-export-of-png-images-from-large-avi-file-with-mathematica-11-3
  [2]: https://mathematica.stackexchange.com/users/9239/piotr-wendykier