Description of problem:
I was writing using the IME, tried to write う and pressed tab to se the list of suggestions, then backspace to go back.

Version-Release number of selected component:
ibus-anthy-python-1.5.12-7.fc34

Additional info:
reporter:       libreport-2.15.2
cmdline:        python3 /usr/share/ibus-anthy/engine/main.py --ibus
crash_function: __update_anthy_convert_chars
exception_type: TypeError
executable:     /usr/share/ibus-anthy/engine/main.py
interpreter:    python3-3.9.6-2.fc34.x86_64
kernel:         5.13.7-200.fc34.x86_64
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
engine.py:1278:__update_anthy_convert_chars:TypeError: can only concatenate str (not "NoneType") to str

Traceback (most recent call last):
  File "/usr/share/ibus-anthy/engine/engine.py", line 1299, in __update
    self.__update_convert_chars()
  File "/usr/share/ibus-anthy/engine/engine.py", line 1222, in __update_convert_chars
    self.__update_anthy_convert_chars()
  File "/usr/share/ibus-anthy/engine/engine.py", line 1278, in __update_anthy_convert_chars
    self.__convert_chars += text
TypeError: can only concatenate str (not "NoneType") to str

Local variables in innermost frame:
self: <engine.Engine object at 0x7fba89694940 (engine+Engine at 0x56112e547170)>
pos: 0
i: 0
seg_index: -1
text: None