This error happens because FP6 adds a new function to the Quad Menu. It's added by the C:\3dsmax2018\3ds Max 2018\scripts\Startup\forestpack.ms script.
Although the script is removed when FP6 is uninstalled, Max keeps a cached copy of the Macroscripts created by forestpack.ms.
Please, try closing Max and deleting all 'iToo', 'ForestPack' and 'RailClone' .mcr files from this folder:
C:\Users\<username>\AppData\Local\Autodesk\3dsmax\2018 - 64bit\ENU\usermacros (replace the '2018 - 64 bit' for your Max version).
That should fix the problem.