When there are multiple DFFs and one of them is removed, its reference lingers inside bit2driver dict. While invoking handle_dff() function for other DFFs, this broken reference is used isnside sat_import_cell() function.
When there are multiple DFFs and one of them is removed, its reference lingers inside bit2driver dict. While invoking handle_dff() function for other DFFs, this broken reference is used isnside sat_import_cell() function.