rendering float problem : TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'
When recording a Move, when typing the amount, the following error occurs:
```
File "/usr/local/tryton-tip/lib/python3.9/site-packages/tryton/common/cellrendererfloat.py", line 41, in key_press_event
buffer_.delete_text(start_pos, end_pos - start_pos)
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'
```
The error popups when typing the '.' character. For reference, I am using french locale, and use the numpad for decimal.