Thanks to Quipyowert for finding and fixing this bug!
struct ElementInfo *ei = &element_info[element];
struct ElementGroupInfo *group = ei->group;
struct ElementInfo *ei = &element_info[element];
struct ElementGroupInfo *group = ei->group;
+ if (group == NULL)
+ return -1;
+
xx_ei = *ei; // copy element data into temporary buffer
xx_group = *group; // copy group data into temporary buffer
xx_ei = *ei; // copy element data into temporary buffer
xx_group = *group; // copy group data into temporary buffer