Same problem, Nalincah's solution worked for me! To summarize for future readers:
Problem: B9 Part Switch throws fatal error on launch, saying something about "ht2_02: part does not exist"
Cause: When using Habtech2 and Kerbalism together, a line in the Kerbalism config can cause B9 to attempt to load a nonexistant Habtech part.
Solution: Go to your Kerbalism config folder, find the file HabTech2.cfg; look for the line starting with "-B9_TANK_TYPE[ht2_02]:..." (for us, using the default config setup, it was line 1790) and comment out this line by putting // at the beginning.
