The file thermodynamicInfo.txt contains an extra line END which the MFiX GUI is not expecting. The GUI is trying to pop a warning about this extra line at the end of the file, but there is an error in the code which is responsible for the warning.
You can safely ignore this warning (press Cancel) Or you can remove the line END from thermodynamicInfo.txt.
We will be releasing MFiX 25.2 in the next few weeks and this problem should be fixed. Sorry for the trouble.