Did you refer to install Sothink SWF Decompiler for Mac V4.5?
We suggest you open the terminal, and then input the following two command language:
ls > ~/sothink_name.txt
Then, you will be directed to your user home directory, and open sothink_name.txt file under this directory, and then delete all the information regarding swfdec_Mac or Sothink SWF Decompiler for Mac.
Lastly, please reinstall Sothink SWF Decompiler for Mac 4.5 for a try.
If the above does not help, please provide the sothink_name.txt for us to have a check.
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Have a nice day!