Had this file corruption occur in the current beta D88.
- Add an integral
- Right click and select 'Toggle var binding'
- Delete the variable attached to the integral
- Hit 'Toggle var binding' again
- Save and exit
- Upon reloading, it gives an error "Variable '' is invalid for this position" (omitting the variable name)
I was able to recover the mky, by opening it up (EDIT: In a text editor) and manually removing the bad variable (easy to spot as it had no name).