In WideCharToChar in UEncodings the 1st call to WideCharToMultiByte passes reference to Dest array, before that array's size has been set. Instead of passing Dest, nil should be passed here.

Instead of

  BufSize := WideCharToMultiByte(
    CodePage, 0, @Source, 1, @Dest[0], 0, nil, nil
  );

uses

  BufSize := WideCharToMultiByte(
    CodePage, 0, @Source, 1, nil, 0, nil, nil
  );