🐛 Bug report | [LogFile deformation gradients error]

**Description:**
the log file cannot output deformation gradients. I can obtain F from plotfile. 

**Version:**
latest version. 
**Steps To Reproduce:**
1. Go to 'log file'
2. Click on 'export Fxx; Fxy; Fxz.....'
3. Run feb
4. See error: either crush or error message. 