Crash tgkill with a weird weird record and no activity

Question

I recently fixed all nullpointer services in my application, and it has about 100 daily users, 2k-month users.

Today I have a really weird crash report. Where does he usually say

java.lang.Exception
com.package.app.activity

he just says tgkill. Only one user crashes 2 times. It is on android 6.0 and has Galaxy Note4 (trelte). Further below, where it is usually said:

java.lang.Exception: 
  at com.package.app.MainActivity.a (Unknown Source) 
  further lines

He says:

backtrace:
  native: pc 000000000004276c  /system/lib/libc.so (tgkill+12)
  native: pc 0000000000040379  /system/lib/libc.so (pthread_kill+32)
  native: pc 000000000001ca9b  /system/lib/libc.so (raise+10)
  native: pc 0000000000019d19  /system/lib/libc.so (__libc_android_abort+34)
  native: pc 000000000001755c  /system/lib/libc.so (abort+4)
  native: pc 0000000000008727  /system/lib/libcutils.so (__android_log_assert+86)
  native: pc 0000000000020201  /system/lib/libhwui.so
  native: pc 000000000001d4c9  /system/lib/libhwui.so
  native: pc 000000000001f20b  /system/lib/libhwui.so
  native: pc 0000000000022301  /system/lib/libhwui.so (_ZN7android10uirenderer12renderthread12RenderThread10threadLoopEv+80)
  native: pc 0000000000010205  /system/lib/libutils.so (_ZN7android6Thread11_threadLoopEPv+112)
  native: pc 0000000000062b23  /system/lib/libandroid_runtime.so (_ZN7android14AndroidRuntime15javaThreadShellEPv+70)
  native: pc 000000000003fc7b  /system/lib/libc.so (_ZL15__pthread_startPv+30)
  native: pc 000000000001a39b  /system/lib/libc.so (__start_thread+6)

What could it be?