NPE problem Solved:
Using information from Java Error in Group Chat Permissions
I noticed 2 problems for group in question.
Problem 1: ofMucMember table has 36 users while room maximum was set to 30. I increased room size but NPE problem persist.
Problem 2: Some user’s names were misspelled.
Initially I tried to delete misspelled names from database, but that did not help.
Correct solution was to edit user names in table. Since some users had misspelled and properly spelled names, I use some random existing user (not from Owners or Administrators)
After restart Permission tab start working, and I was able to delete “modified” users using console.