stringJoin error in Mathematica for MG5 conversion

I am encountered the world of Mathematica and FeynRule quite recently and try to implement a Lagrangian including the Top Partner(BSM) fields in addition to the Standard Model. In the process, everything worked fine until I find the error in making UFO files for MG5.

StringJoin::string: String expected at position 2 in P.<>CreateObjectParticleName[PartNameMG[Overscript[topT, -]]]. StringJoin::string: String expected at position 2 in P.<>CreateObjectParticleName[PartNameMG[Overscript[topT, -]]]. StringJoin::string: String expected at position 2 in P.<>CreateObjectParticleName[PartNameMG[topT]]. General::stop: Further output of StringJoin::string will be suppressed during this calculation.

In this error message, my understanding is that in the process of making the python files which are usable for MG5, Mathematica cannot convert the topT (in my case, one of the Top Partner fields) into string. I would like to know how to fix this problem. I have tried to run MG5 with this UFO output file, but it immediately complains that the above line gives a syntax error in vertices.py file. I would really appreciate if someone could help me.

For additional information, I am using Mathematica version 11.3.0.0 and FeynRule version 2.3.32.

Warning: Non positive interaction order QED.

I also get this error in the end even if I set the M$InteractionOrderLimit to be lower than equal to 99 for both QED and QCD.