cell of FORMULA type invoke error in xlsx by Excel 2019

```lua
C:\xxx\lua53.exe: .\xlsx.lua:109: attempt to index a nil value (field '?')
stack traceback:
	.\xlsx.lua:109: in method '__load'
	.\xlsx.lua:308: in function 'xlsx.Workbook'
	...s\xxx\luaplus\Src\Modules\xlsx\tests\printxlsx.lua:6: in main chunk
	[C]: in ?
```
Which cell content is '=1+1'.